See announcement here, FAQ here.
In a nutshell: You'll be able to trade games and in-game items (right now: TF2 items; soon: Portal 2 stuff; in the near future: items from compliant third-party games, too) with other Steam users. Note: It won't enable you to get rid of a game you've bought for yourself (i.e. 'used') - it's for games in your gift basket. If you've received a game through trading, you can keep on trading or 'open' it for yourself.
Any thoughts? I guess it's ... interesting. No complaints since it won't affect me if I have no intention of using that feature. If anything, it gives me an alternative way to get ahold of games that might be completely overpriced or not available over here, e.g. older id Software games that don't happen to be Commander Keen.
So, I'd assume I'm not the only one to think that this might be just the first step for Valve to ultimately turn this into an RMT platform that will also enable you to sell/buy stuff, too? Possible short-term effect: Sales generated through special Steam deals--holidays, daily deals, weekend deals--could increase because people want to load up their gift basket.