So to get this working on Win 7 64 was a bitch. Took me about two hours. Here's the steps I took to finally get it to work:
1) Set Steam to run in compatibility mode (I used XP SP3). Don't worry, this will get set back. 2) Launch steam in admin mode. 3) Run Arma II base game. If you get an error about license key, it's not writing the registry key. Make sure you're in admin mode and compatibility mode. 4) Run Operation Arrowhead. Same deal. You want it to launch with no key issues or anything else it complains about.
At this point, you could open up regedit and go to the wow6432node. You should see an entry or two for Bohemia. You can try to launch Combined Operations now. If you get any errors, delete the Bohemia entries in the registry and try again.
After you get it working, you can turn off compatibility mode and not launch as admin. Getting DayZ itself to run is pretty easy after that if you use the video guidelines (I think they were posted above).