IN theory yes.....
BUT and a big BUT
For starters MS servers MS want users to pay to access them. This would mean non paying users getting access.
Also PSN and xbox live is different setups so connecting them together would mean a new system just for one game. And MS won't allow that as it provides it all.
PSN it down to the devs for the online so they can do there own stuff almost.
PC and PS3 users most of the time are pretty close when it comes to servers just a couple lines of codes stops them from joining together. Some games use the same leaderboard servers or something
PSN more likely to have it happen with it against PC.
XBL too closed off sadly

think that right