That's not true, actually. Both the Xenon and the PPU are PowerPC derivatives and run big-endian. http://blogs.msdn.com/robunoki/archive/2006/04/05/568737.aspx.

360 and PS3 are both PowerPC derivatives but they're not FPU identical (esp when you add in the SPUs!), which makes cross console MP impossible for games that rely on a deterministic simulation w/ input passing (most RTS, sports, and fighting games). Client/server games are a possibility, though as the previous poster sez most games rely on native matchmaking and NAT traversal services which are definitely not cross platform. And even if all that were true, Xbox Live is a walled garden. Games need special permission if they want to reach outside of Live and talk to the real internets.