I don't see why it shouldn't be possible, as whilst they deal with things differently client-side (e.g. how the graphics are rendered, how they deal with network protocols), as long as they abide by the same basic networking protocols like TCP and UDP that are used for networked gaming, and they keep to the same standard of message transfer to communicate what's happening in a network game, then it's all fine. These standards may have to be implemented in different ways on each platform, but as long as they are adopted in each case networked gaming is perfectly possible, hence why most multi-platform games support multiplayer.
I have played Zynga Texas Hold'em Poker on Facebook and on my iPod, and it does not matter which client you use, you can play on a table where both clients are in use. Both are receiving and sending the same data (pretty much), only the way the user interacts with them is different.
You might be interested to know that
Civ IV if purchased on Steam works on both PC *and* Mac now!