I saw COD4 stretched across three screens and Fry's the other day. The resolution wasn't set right, so everything was stretched and it ran like crap. Don't know what hardware it was running on though.
^ That's probably the case for most games. I had a friend of mine run Halo on my computer stretched out across two monitors. Wasn't very playable at all.
I think you can with Bioshock over 3 monitors. Microsoft Flight sim can span multiple monitors also, which honestly would enhance the experience.
I have a few monitors and in my experience, PC gaming is best with just 1 nice monitor attached to the computer, unless you are doing the above mentioned. Even having 2 monitors was a problem when playing FPS's just on 1 screen. I know there is software, workarounds, etc, but it just isnt worth it. Also, I've heard it makes some people motion sick.
I like it for WoW though, because I could be grinding and watching a movie/browsing the web at the same time.