http://www.teamfortress.com/post.php?id=6040

It's on the TF2 blog but it applies to Steam as a whole.

It's a neat concept: You can now trade items from numerous games as well as games themselves. Currently the beta only supports TF2 items and Steam games, but they plan to add Portal 2 into the mix soon and are already contacting other devs to see if they want to be a part of it.

With this we can now trade TF2 items without ever launching the game, can trade items from different games for each other (For example, I can trade you a co-op skin in Portal 2 for a hat in TF2 and the trade would automatically happen in the Steam Cloud) or even trade in-game items for actual Steam games, if you can find someone willing to make the trade.