Olivier Langlois's blog

Some people are maybe wondering why we are not seeing cross platforms titles such as Guitar Hero 3 being compatible from one version of the game to the other versions. For instance, I have the Nintendo Wii version of GH3 and I would have loved to be able to play with my coworkers online that have the PS3 version.

Technically, it is possible because, with the exception of Microsoft Xbox consoles that are using a proprietary encrypted protocol, all consoles are using the same standard TCP/IP protocols to communicate and TCP/IP protocols goal is to connect different hardware together. Usually, game developers are either using a network engine that adds another protocol on top of UDP or they develop themselves the new protocol layer but either way, the protocol will be identical from one platform to the other.

When I was working at Quazal I have frequently let network test programs communicate between a Xbox 360 (in development, you can turn off the proprietary encryption to let you debug your programs) and PS3s flawlessly.

That would be cool to have that feature in retail games but the reason we do not see it is only because it is against the console makers interests who have full control on the software sold for their respective platform. You want to play online with your friend that just purchased a new Xbox 360 game. Well, you will have to buy a Xbox 360 too or else you will not play online with your friend. Period.