No, you still can't trade used digital games for cash or credit, but a new Steam exchange beta of sorts promises to let you pass around just about anything else. The beta's live now, and all you have to do to participate is launch the client (both Mac and PC versions are supported), click on "Change" under the "Beta Participation" subsection, and opt into the "Steam Trading Beta," prompting the client to pull down a roughly 38MB update, install, and relaunch.
Thereafter, you'll be able to trade Team Fortress 2 items and Steam Gifts. The former's a free-to-play first-person team shooter that already offers game-related item trading (as of September 30, 2010), while the latter involves anything purchased under Steam's "gift" label, including full games (so long as they've not been "unlocked," i.e. added to your game library).
The September 2010 TF2 update, dubbed "the Mann-Conomy update" with its introduction of the Mann Co. Store, added 65 new items to the game, including hats, weapons, mini-games, and tools, as well as the option to buy items directly (via micro-transactions) and trade items with other players.
The "Steam Trading Beta" introduces Steam Gifts, and promises "both Trading and Inventories" will be added to current and future games soon. The Steam Trading Beta FAQ suggests Portal 2 is next up, and "should be reasonably soon," with third-party games to follow "in the next few months."
If you have ideas for other bits and bobs you'd like to trade via Steam (*cough* used games *cough*) there's a feedback forum to sound off. Trafficking in used games—a service that's essentially keeping brick and mortar retailers like GameStop alive—may be a holy grail for digital. Finding the right balance to make such a feature financially attractive to gamers as well as profitable for service providers like Valve—say via auction houses with transaction fees, transaction surcharges in direct sales and swaps, or digital exchanges for store credit—is the trick.