A cross platform multiplayer game between Xbox 360, PS3, and Wii would pprobably be possible, only if they all used the same exact online service, which they do not. The controllers are also drastically different, so a Wii/PC plyer would demolish a 360/PS3 player in a FPS. And then there are the differences in tech specs, and lets face it, none of them are going to partner with each other (the last time Sony partnered with Nintendo was for a CD add-on for the SNES, but that didn't work out, and led to the creation of the Playstation).
So to put it simply, NO, its not possible.
[Edited on 11.16.2010 2:08 PM PST]