|QUOTE (fullmetal @ Jan 2 2010, 07:04)|
I thought it was only possible with Valve games, like you mentioned.
I didn't know it was possible so my apology for having it wrong. My guess is that it's only possible with games that have full Steam support (Steam Cloud support/Achievements in Steam/...).
Doubt it would work for, let's say, Call of Duty 2 (bought at retailers years ago). Or with Call of Duty 4/World at War (also bought at retailers).
List of retail games that can be activated in Steam