In Dark Souls, can player from different platforms interact with each other?

My friends own the PS3 version of the game, while I am playing on the Xbox 360. Will we be able to join each others' games?

No, Sony & Microsoft don't want to share. This isn't a technical issue (examples: Shadowrun for Xbox 360 can play with the Windows version, Portal 2 supports the PlayStation 3, Mac OS X, Linux and Windows versions playing together), but is a very deliberate business decision on their parts to try and carve out as much of the userbase for their own exclusive use. Even in cases where companies run their own private servers outside of XBL and PSN, they don't let the users from one sandbox play with the users from another (even though essentially it is the same clients talking to the same server using the same protocol).

TL;DR

Microsoft and Sony are kings of their respective sandboxes and won't let anyone play a game with anyone in another sandbox.