From this article on Tom's Hardware, there's this interesting snippet found in a Microsoft presentation on XNA:
"One slide of particular interest in Microsoft's presentation hinted at a PC gamer playing online against Xbox gamers."
Of course that makes perfect sense and seems to be a no-brainer. It expands the player base and brings in more revenue. It also provides further differentiation with Sony. And since the underlying technology isn't radically different, it's not so hard for a game developer to support both platforms. I think you can count on this happening.
On the Microsoft XNA FAQ, there's also this statement:
"Microsoft XNA lays the groundwork for enhanced, more unified experiences, including a common controller that can be shared between the Windows PC and Xbox."
So gamers on both PC's and Xbox might be using the same controller too. More evidence that they'll probably be playing together. Speaking of controllers, check out the DX1 from Ergodex. Great idea.