Okay, thought I'd redeem my smart ass, got this off the web:
The main difference is that each version will only work for a specific game platform. For example, an Xbox 360 version of Guitar Hero 3 will only work on the Xbox 360 -- not the Wii, Playstation 3, and so on. A Wii version will only work for the Wii ... etc.
The versions for Xbox 360 and Playstation 3 will have better graphics than the versions for Playstation 2 and the Wii since those platforms are optimized for high-definition televisions (the Wii is not -- neither is the Playstation 2).
Also, owners of the Xbox 360 will have the option of downloading more songs (and other content) via Xbox Live.
Other than that, no difference. Exactly the same.