I just have to have it. In my previous Windows life, I had no problem setting up Windows XP and Vista with a NEC MultiSync 90GX2, and a BenQ FP202W. I don't want to buy new stuff just to get the same amount of screen real estate.
It was a little harder than I expected. Took some command line rescues and online searching. Eventually the config working for me is like this. Since the graphics core in my laptop (Latitude E6400) is Nvidia Quatro NVS, the tool to use is nvidia-settings coming with the Nvidia binary driver. I used separated X screens with Xinerama enable. Both screens have auto resolution. That gives me the ability to freely move windows across the two monitors. I can still use multiple workspaces too.
I like my Compiz eye candies, but there's no way to get them working. Some suggested using twinview with no Xinerama. It didn't work for me. Well, it did, only for a minute or so after logging in. Compiz effects worked, but then the whole computer would lock up solid, requiring power button treatment.
If I use the laptop by itself, Nvidia correctly falls back to one screen setup on LCD. And all Compiz stuff will be alive again, which is a nice bonus.
I've yet to find a way to use 3 screen setup, with laptop LCD in the mix. But it should definitely be possible.