I managed to get a corrupted save on my copy so thought i would post here to perhaps help people avoid it. I normally dont connect to live when playing offline, im sure there are others who do the same (annoying friends online, downloading other stuff etc).
Ok, so i started out playing offline co-op for a good four chapters of act 1. Saved and quit and then thought ill have a little play online. Signed in to live and straight away got a game update request. Did this, game restarts and got all my stats updated for the first two games, beta etc. Then when i press start i get "Your current save game is corrupted press A to delete and start again" or words to that effect. So basically lost a couple of hours game play.
So if you just got the game, or are about to, make sure you do the update download before starting the offline campaign.
On a plus note, team deathmatch was good!