Currently, Battlefield 3 does not support cross-platform multiplayer.
According to this article, which quotes the studio head, Robert Bach, there are numerous differences between the PC and Console versions of Battlefield 3:
Limitations caused by hardware requirements causes the maximum number of players in a game to vary between versions:
The biggest difference between the PC and console version of Battlefield 3 is that we have 64 players on the PC and 24 players maximum on console...We tried to get more players in [to the console version] but then you need to scale down all the graphics, scale down all the destructibility, and sometimes you need to scale down all the map sizes
Alterations were also done on the maps themselves to make them work better for smaller numbers of players:
When we say they are smaller, it's not that we have cut them in half. But we tried to compact them slightly to keep the action up.
Judging by these differences, and the licensing/requirements mentioned by Stof, it is extremely unlikely that there will be support for such a feature.
On a side note, having such a feature would be a huge win for this game franchise, which is in constant competition with the folks over at Call of Duty, and I'd hope that any game promoter/marketer would have immediately jumped on it as a major selling point if it was a plausible future enhancement.