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Kotaku
Note this isn't happening to everyone, just a select, lucky few, but it looks like the new Crackdown downloadable content is erasing some people's save game data (progress, orbs, etc). The problem's arising from co-op games where the host has the Keys to the City cheat mode enabled, which is resetting some people's data. So long as you don't join a co-op game where the host has this mode enabled, you should be fine.
Realtime are currently working to fix the problem. In the meantime, be a good kid and avoid co-op with strangers. If you don't do that already.
EDIT: There's a temporary workaround for this problem. If you do the following:
1. Sign-in to Xbox Live
2. Start Crackdown and receive new Title Update (if you haven't already)
3. Begin a Single Player Campaign game
4. Once loaded, press Start and choose "Quit"
5. Reboot Crackdown (soft reboot or hard reboot of console)
You should be OK.
Official Crackdown Forums [Thanks Owen!]
