Multiple monitors is really handy for GUI development. Particularly on Windows. Visual Studio eats up tons of space, so your program overlaps the code you are stepping through, leading to a lot of repaint messages that can make tracing through code tedious. Plus, windows can't be moved, resized, minimized, or otherwise interacted with until Windows can call their wndproc. Debugging an always-on-top window (like a menu) is a pain.
Multiple monitors is less effective on Linux. X can unmap, move, etc. windows without their permission, so virtual desktops still work when a program is being debugged (like when you need to look up documentation). Plus, emacs is pretty slim, visually, so you can often fit your program right next to it. 'Course, if you use eclipse, it sucks up extra space like VS.
(Vista might be able to move windows without their permission, since the contents are redirected to a buffer. I don't have vista to try.)
I like using multiple monitors largely for the aforementioned reason that you can have code on one screen and documentation / web page output / forum discussions / IRC on another. In my opinion it is far easier to context switch by simply looking left and right as opposed to continuously switching between windows being in the foreground or background. I also think it's faster to look around than alt-tab, plus you don't lose your spot on the page you're reading / document you're writing as easily when compared to having the text completely disappear from view after switching window focus on a single monitor setup.
Multiple monitors is less effective on Linux. X can unmap, move, etc. windows without their permission, so virtual desktops still work when a program is being debugged (like when you need to look up documentation). Plus, emacs is pretty slim, visually, so you can often fit your program right next to it. 'Course, if you use eclipse, it sucks up extra space like VS.
(Vista might be able to move windows without their permission, since the contents are redirected to a buffer. I don't have vista to try.)