I have had this problem for a while now.. I have searched and searched the Internet, followed all kinds of leads, all to no avail..
I went to Login Window from System > Admin and turned on Debug... The result is as follows, any ideas and this is just an excerpt.... the whole thing is monstrous..
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Sending command: 'GET_CONFIG greeter/PreFetchProgram :0'
Dec 7 11:01:23 tatanka-laptop gdm[5654]: DEBUG: Handling user message: 'GET_CONFIG greeter/PreFetchProgram :0'
Dec 7 11:01:23 tatanka-laptop gdm[5654]: DEBUG: Handling GET_CONFIG: greeter/PreFetchProgram for display :0
Dec 7 11:01:23 tatanka-laptop gdm[5654]: DEBUG: Looking up per display value for greeter/PreFetchProgram
Dec 7 11:01:23 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: greeter/PreFetchProgram
Dec 7 11:01:23 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: greeter/PreFetchProgram
Dec 7 11:01:23 tatanka-laptop gdm[5654]: DEBUG: Requesting group=greeter key=PreFetchProgram locale=(null)
Dec 7 11:01:23 tatanka-laptop gdm[5654]: DEBUG: Looking up key: greeter/PreFetchProgram
Dec 7 11:01:23 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: greeter/PreFetchProgram
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Got response: 'OK /usr/lib/gdmprefetch @/etc/gdm/gdmprefetchlist'
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Sending command: 'CLOSE'
Dec 7 11:01:23 tatanka-laptop gdm[5654]: DEBUG: Handling user message: 'CLOSE'
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Hidden=false
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/TryExec
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Exec
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Name
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Comment
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Hidden=false
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/TryExec
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Exec
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Name
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Comment
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Hidden=false
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/TryExec
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Exec
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Name
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: Desktop Entry/Comment
Dec 7 11:01:23 tatanka-laptop gdmgreeter[3323]: DEBUG: Attempting to parse key string: GdmGreeterTheme/Greeter
Dec 7 11:01:32 tatanka-laptop gdm[5655]: WARNING: Couldn't authenticate user
Dec 7 11:01:38 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: debug/Enable=false
Dec 7 11:01:38 tatanka-laptop gdm[5654]: DEBUG: Handling message: 'QUERYLOGIN 5655 tatanka'
Dec 7 11:01:38 tatanka-laptop gdm[5654]: DEBUG: Got QUERYLOGIN tatanka
Dec 7 11:01:38 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: debug/Enable=false
Dec 7 11:01:38 tatanka-laptop gdm[5654]: DEBUG: Handling message: 'LOGGED_IN 5655 1'
Dec 7 11:01:38 tatanka-laptop gdm[5654]: DEBUG: Got logged in == TRUE
Dec 7 11:01:38 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: debug/Enable=false
Dec 7 11:01:38 tatanka-laptop gdm[5654]: DEBUG: Handling message: 'LOGIN 5655 tatanka'
Dec 7 11:01:38 tatanka-laptop gdm[5654]: DEBUG: Got LOGIN == tatanka
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: debug/Enable=false
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Handling message: 'GREETPID 5655 0'
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Got GREETPID == 0
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: debug/Enable=false
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Handling message: 'WRITE_X_SERVERS 5655 0'
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: daemon/ServAuthDir=/var/lib/gdm
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: debug/Enable=false
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Handling message: 'SESSPID 5655 3345'
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Got SESSPID == 3345
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: gdm_socket_handler: Accepting new connection fd 8
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Handling user message: 'VERSION'
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Handling user message: 'GET_CONFIG greeter/GraphicalThemedColor :0'
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Handling GET_CONFIG: greeter/GraphicalThemedColor for display :0
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Looking up per display value for greeter/GraphicalThemedColor
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: greeter/GraphicalThemedColor
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: greeter/GraphicalThemedColor
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Requesting group=greeter key=GraphicalThemedColor locale=(null)
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Looking up key: greeter/GraphicalThemedColor
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: greeter/GraphicalThemedColor
Dec 7 11:01:43 tatanka-laptop gdm[5654]: DEBUG: Handling user message: 'CLOSE'
Dec 7 11:01:44 tatanka-laptop gdm[5654]: DEBUG: Attempting to parse key string: debug/Enable=false
Dec 7 11:01:44 tatanka-laptop gdm[5654]: DEBUG: Handling message: 'opcode=SHOW_ERROR_DIALOG$$pid=5655$$type=1$$error _msg=Your session only lasted less than 10 seconds. If you have not logged out yourself, this could mean that there is some installation problem or that you may be out of diskspace. Try logging in with one of the failsafe sessions to see if you can fix this problem.$$details_label=View details (~/.xsession-errors file)$$details_file=/home/tatanka/.xsession-errors$$uid=0$$gid=0'

But as you can see, the errmsg is shown.. Hope someone can be of help.. Its not critical, just annoying as all get out.

I used to get this error message every once in a while when running Fedora & Gnome.

The only solution I came up with (or discovered), was to move, so that you may refer to it later), the ~/.gnome2 folder.

I'm doing this from memory, since I use neither gnome nor Fedora anymore, but since you're moving the folder, if this doesn't solve your issue, you can restore it.

cheers,

As Charlton Heston once said... Ben Hur, Done That.. i tried renaming .gnome2 to .Hgnome2 and no luck.. still same problem.
Oh well, such is life.
Thanks

I had a similar problem, I just disabled my firewall, and bingo it worked.

Gardas.. Nice try.. ive just about given up.. maybe ill try KDE desktop although i dont think that will help, but worth effort i think...
Thanks everyone for your help.. Kinda gets to me that even with the Debug for login enabled, you still cant see why this is happening.

Im good.. I thnk.. I installed KDE 4.1 and ran that desktop for a bit...Then i rebooted and just before the login screen the system complained about my video not being configured correctly.. Dont recall exactly what i did, but it kinda hung. Rebooted and then up came the KDE login splash screen.. I clicked on what i thought was the KDE Desktop and logged in.. Lo and Behold, It's Alive, It's Alive.. My Gnome is back. Well, dont know why it's working but im not gonna look a gift horse in the moutn..
Thanks again guys.

I have the same problem before, and I can not login using xfce.
As I remember I use the command "chmod 777 /tmp/" to solve this problem.