Just an FYI, I didn't see anyone post this and searched for it as well. Just throwing it out there, sorry if it is a dupe.
Also, do not 'reinstall' your game. Just use these simple steps and it will save you hours of downloading, and it does the exact same thing.
1. Start Steam in Administrator Mode
2. Right click on your ARMA 2 and click Properties
3. Click the Local Files tab
4. Click 'Verify Integrity of Game Cache'
5. Repeat step 4 for "ARMA2: Operation Arrowhead"
6. Run both games once with Steam still being ran in administrator mode.
7. Update your DayZ if need be, and play!
Edited by gair22, 20 August 2012 - 10:44 AM.