Though reports vary somewhat, the problem appears to affect players who participate in the game's online co-op mode and then return to single player. "Agent G," a staffer for developer Realtime Worlds had this to say in the forum: "hey guys - just so you know, we are aware of the situation and working on it." Hopefully we'll hear something soon, but as there's no guarantee you'll be able to retrieve your old data, we're calling this a case of "downloader beware" at the moment. Stay tuned.
Update: Realtime Worlds has detailed a method (posted after the break) for preventing the problem until the game receives its next update. If you've already been struck by save file calamity, however, there's not much you can do besides scold the Xbox Live certification process.