I would imagine that there won't be cross over on all games.
Battlefield will always have dedicated servers for PC clients and PC players will stick to that.
Stuff like CoD with client hosting should have settings to keep console players away from PC players. It's perfectly acceptable to be able to detect which platform you're playing on and just have the option to search for 'console only' games.
For racing and sport games this will be great. As it will be for coop gameplay on things like resident evil 5 for example.