I was reading this thread as I too have the same problem but (seemingly) with all versions of Ubuntu (10.04, 10.10, 11.04).
I have found out the solution to this problem and registered on this site just to post it.
So, after not seeing any 'alongside..' option and being dissatisfied with the advanced 'something else' option, I tried to see what was going on by launching GParted Partition Editor from the live cd/usb(I had 10.04 and 11.04 on cd and 10.10 on usb).
I had noticed immediately that the windows partition that I wanted to install alongside
had a little red icon with an exclamation point in it.
Naturally, I right-clicked for some details and got a huge confusing list of errors.. something about clusters.
Anyway, in all that mess are the instructions! It'll say something like "in windows run 'chkdsk /f' and then restart TWICE". (chkdsk = Check Disk for errors, /f = Fix errors automatically)
SO, boot into windows. Whatever version you have(I am working with vista dual boots, but check your partition in GParted it's probably the same).
Double click 'My Computer'. Right click on the hard drive you want to be dual booting with and click 'properties'.
Go to the 'tools' tab. The first option should be to check drive for errors. Click this and make sure
the option to automatically fix errors is checked. You can check everything to be sure.
After clicking 'check' or 'ok' you'll get a pop-up that says the check cannot be performed while the disk is in use, from here you can schedule it to check on the next startup. Do so then restart
Once restarting...yea its gonna take a while..you go through that checking process but be glad when it's done.
After it has checked and fixed your drive and your booted up (do not have to be logged in)... restart
, just a plain ol' restart with no checks.
AGAIN, booted up into windows... RESTART
(yes it really seems to be required a second time after the check.)
Finally, booted into windows for the final time, and (you can put Ubuntu disc in if you want) ...SHUTDOWN
Boot into Ubuntu and run the installer(I ran 'try ubuntu' and then the installer from there) VOILA!
Alongside with them should now be an option.
I am now using it as we speak for 11.04.
Turns out it because I've booted into live CDs and USBs beforehand and expected the ntfs filesystem to stay intact.
This explains why people in the install video can put in a fresh cd into a fresh system and have no problems, while other like me have been trying ubuntu for a while before deciding to install it.
Anyway, windows has to check and fix and messed up clusters. Then restart into the fresh error-free drive, and once more (it has something to do with the ntfs partition itself and how it boots).
Hoped this has helped as much as it has helped me.
P.S. sorry if I'm a little confusing in my rambles..