For those who immediately go "cross-platform is impossible dude", it isn't. It's just very hard to set up right, but once you got it down you can play with anyone on any console as long as they got the same game.
How this is possible? It's called XLink Kai, a network like Xbox LIVE, Steam and PSN but made entirely by volunteers. Because of this, XLink Kai isn't bound to any company nor platform, but can be used for every platform (PS2, PS3, Xbox, Xbox 360, PSP,...). This allows cross-platform gaming. XLink Kai became very popular when PSN went down, as people with a PS3 could still play online through XLink Kai.
XLink Kai operates through your PC. Tutorials on how to connect your console with your PC and then to XLink Kai can be found on their website.
NOTE: XLink Kai is very sensitive for bad internet connection. Do a Ping test, and if it's above 35ms, don't even bother
The reason I'm posting this is because I wonder if it wil work with an AC game. However, XLink Kai requires wired internet for the PC which I don't have. If anyone is willing to test this, it may allow people from PC, Xbox 360 and PS3 to play together.
Just thought I'd let you guys know since someone started a thread about cross platform gaming for AC III. I think this will work for ACB but not ACR (due to the UPlay Passport).
If anyone can confirm this works, let us know here!