There was a single player demo, but no multiplayer demo for COD4. The single player demo showed much of what the multiplayer gameplay was like though, imo. Nade spam, nade spam, kill lots, die lots, start the next round/level.
I wouldn't mind seeing a FC2 demo myself. I enjoyed Far Cry, but dislike the arcade style of Crysis. It'd would be nice to see if they learned from their mistakes Crysis or just made a similar simulator in an alternate environment. Hopefully they'll come out with one after the release.