Valve has an FAQ up covering everything that might need to be covered but the main fact is that you can now trade your in game TF2 hat for a game! Imagine that, hats for games! an astounding proposition!
Valve hope to expand it to Portal 2 and eventually third party games.
This seems to be the natural evolution of all the trading and crafting stuff in TF2. People could do this kind of stuff before but now Valve is making it easier than ever. I have to say the mere idea of trading hats for games sounds crazy but it's exactly the sort of thing I would expect from Valve.