I popped Revelations in just to reacquaint myself with AC mechanics (and reacquire proper inverted camera skills after being stuck with non-inverted with Deus Ex) and was forced to update the game -- after updating, I had all kinds of problems.
Namely, eagle vision doesn't seem to work anymore. The triangle button (your "head"; not sure what the Xbox equivalent is) has nothing assigned to it, and holding it down does nothing.
The other was that multiplayer wasn't available. This was at about 17:00 Japanese time, or 4 AM ET, or 9 PM (?) Greenwich. Could just be a maintenance outage.
Also -- I wouldn't mind playing some of the Revelations DLC that I never got around to, but it costs a ton! $10 each for the Mediterranean Traveler and the Desmond-related stuff. Will it go on sale when AC3 comes out, or should I look for a GOTY edition so that I can sell off my original and not have to pay retail price for unsellable DLC?