I guess I'm one of the few people who has tried two screens and not found a benefit... maybe it's because I run so few applications - just Emacs and Firefox most of the time. I did try a two screen setup for a couple of weeks, with Emacs on one, and Firefox on the other. It just didn't do anything for me. I'm either working in Emacs, or I'm looking at the results in Firefox. I don't find I need to do both at the same time.
The lack on Windows of quick keyboard shortcut access to multiple open buffers and virtual desktops make multiple monitors seem important. Navigating by alt-tab and with the mouse is just too slow. In the X11 world the other buffer/virtual desktop/screen of stuff you want to see is one or two key presses away.
I have never felt a big benefit from multiple monitors, or even especially big monitors. 95% of the time I'm working with text, and I can only comprehend one window of text at a time. As long as I can flit around between multiple buffers, tabs, or desktops without my hands leaving the keyboard I'm fine.
I've run Linux forever and always enjoyed two monitors. I use WindowMaker and have have 7 workspaces (Main, Browsing, Communication, Development, Email, Shells, Misc.). Keyboard shortcuts for everything as I despise the mouse.
Most of the time I'm in "Development" and the left monitor is a fullscreen terminal with documentation/revision control/program output and the right is fullscreen gvim.
Nothing is more important to me than resolution though. I can't deal with anything less than 1600x1200 and almost always run 1920x1200 on everything (including laptop).
I didn't look at the date, but when I saw the price listings, I was like, "What!?" I just bought a 22" widescreen for $200. I certainly appreciate all the extra real estate, and yet I can't deny that I could find a use for more. I'm considering plugging my old CRT into my integrated video card, but it produces so much heat compared to a flat panel.
I am a big believer in 2 screens as well. I have a flat panel plugged into my laptop, and keep my project management http://www.streamfocus.com constantly on it while I work so I can easily move between working on the laptop and knowing what to work on on the lcd. :)
Bah. A 30" is vastly superior to two screens. Think about it: with two screens, the valuable screen real-estate right in front of you, where you want the most important things, is instead occupied by 3 inches of BLACK PLASTIC.
I solve that just by having one centered and the other to the side. Soon I'm getting a third screen. You can get 4-5 22" LCDs for the price of one 30".
Perhaps I'm doing it wrong?