Not really. MS said they already tried cross platform with Live and shot it down.
XBL is surrounded by the electronic equivalent of the Great Wall. It wasn't designed to play nicely with the outside world and they're still struggling with that. A huge amount of issues are legacy from when it was designed for the original Xbox, though those are letting up (such as the friends limit being lifted) as they're willing to shut down original Xbox services.
This has caused issues. No fatal ones yet, but a lot of people at Microsoft aren't happy with the limitations. This is a huge reason why you can't browse the internet on the 360 (not that anyone really wants to, trying the Wii browser once or twice is enough to make people buy a $5 cable to just hook their laptop up to the TV.) This is why Unreal Championship on the PS3 allows mods but not on the 360. This is why so many MMOs have been announced for the 360 then either disappeared or been canceled.
Not to say it isn't possible. 1nsane (or the game just like 1sane) did it with the original Xbox. FFXI is another example, same with Shadow Run. But it's difficult to work within all the limitations and every game incorporating it has flopped.
I think the next Xbox will probably feature a complete overhaul around this, as consoles are going to be close enough to PCs to allow this, but I think some in Microsoft still like the simplicity and security that comes from having XBL being a walled-in fiefdom.