I replaced OSX with Linux on my Macbook Pro. When I had OSX in it, I would connect an external monitor at home and it would be automatically detected and used. When I took my laptop with me to school, I the monitor, and everything on the second monitor would move to the laptop monitor. In this way I could use my Macbook Pro at home or at school without rebooting.

This doesn't happen in Linux. How can this be fixed? I'm running xfce in Ubuntu 11.10.

According to Google, the MacBook Pro uses an AMD Radeon card. This is well supported by AMD's own fglrx driver package, which I am guessing you have installed. (If not, I would advise to do so if you are not strongly opposed to having closed source software.) fglrx comes with "Catalyst Control Center" - look for this in your application menu, you should be able to find it there. If not, go to a terminal and type (or aporpos fglrx) to find commands relevant to fglrx. If you still cannot find it, you probably have not installed fglrx, so look for it in your packet manager (synaptic/aptitude). You may find relevant packages with (or, again, search fglrx) in a terminal.

Also, all modern graphics cards drivers support xrandr, which allow you to change monitor setup, resolution and the like during runtime. If you don't care if you have to do a little reading to set things up, this is a much more flexible alternative to the fglrx way. Type in a terminal to read more. Also, there may be some graphical programs that use xrandr for you, but I usually do everything by terminal, so I dont know.

The functionality to use the two monitors (i.e. automatically move windows and the like) should be provided by your window manager, to which the driver supplies the information about your physical layout. As I have never used this, I cannot tell if this works at all. But at the very least, you will be able to use both monitors and have to move the windows around yourself.

My Macbook Pro has an nVidia card. I use the proprietary driver to control it. I don't think it's compatible with xrandr.

I do think it is compatible with xrandr[*], but nVidia has a proprietary tool as well. Just start up nvidia-settings (type it in a terminal), go to 'Display Configuration' and click away (At least that is how things work for my 9600MGT.)
[*]: http://en.gentoo-wiki.com/wiki/X.Org...onitors/Nvidia - This is for gentoo, but most things should be applicable to Ubuntu.