I've been experiencing an issue with Multiplayer where the network would suddenly drop on me mid game. It would happen quite often and sometimes rendered the MP unplayable. Just a few days ago, I decided to clear the cache on my 360 and re-download the latest patch. Since then, I haven't encountered this annoying problem. If you are experiencing this issue, I would recommend that you clear your 360 HDD cache and test the mp out again. It might not work for all of you.....but I suggest that you do try this out.
update: I would also like to add that this fixed an issue where you would get booted out after there was a Retrieving Session notice during MP