Apparently Epic didn"t cover all of their "DRM bases" when they launched the two free Steam week-ends. Anyone who participated in that weekend got a temp key that worked for that weekend.
But there is a problem... Those keys can also be used to install the retail copy of the game. Not only can they be used to install the game, but you can use them to play online pass the free weekend period. Yup, if you were a part of that week-end, you can use that key to play online until Epic figures out what they have done or figure out how to fix it...
Looks like a little bit of karma to Cliff Bleszinski (aka CliffyB), Design Director at Epic, who back in September of 2008, basically accused any PC gamer that can put in a video card to be a pirate.