The PC version of Battlefield Bad Company 2 has a nice handy UI for switching teams but it’s possible to do the same on consoles too. I should have realised this myself but I read it on a comment on this post:
You can do it, press the Xbox guide button in the middle of the controller, find the guy on the opposite team on your recent players list, join session in progress. It’s not easy as sometimes the guy will have set his online status to appear offline, but it is possible.
The Playstation 3 has a “Recently Met” players list too so it should be possible to do it there too.