It seems a lot of people that bought Doom 3 over the weekend on Steam are surprised by a message telling them to enter a CD-KEY before they can actually play the game.
The solution to this is actually very simple. You need to launch Doom 3 ROE (Resurrection of Evil) once before Doom 3 will work. The reason for this (thanks to Alfred Reynolds from Valve Software) is that if you have ROE installed then Doom 3 wants both keys but Steam only gets you a key when you run the game. If you run ROE one and quit then Doom 3 should work just fine.
The Unofficial Solution
Another unofficial solution is to “Go to c:\program files\steam\steamapps\common\doom 3\base and look for this file: xpkey. Open it up in wordpad and copy the key* then close that file out and go to STEAM_xpkey and paste it where you see %CDKEY% and then save the file and then restart Doom 3 it should now work for you.”
Thanks to Alfred from Valve Software and the tip from Steampowered.com.