For now, the only game that supports the trading of items is Team Fortress 2, but Valve says support for Portal 2 will arrive "reasonably soon," and that it plans on offering support for "several third-party games in the next few months." Interested parties can participate by simply heading into the Steam client settings and selecting the option to opt-in to the Steam Trading beta. As participants ourselves, for the next 12 hours we'll be accepting any and all copies of full games for a very special, one-of-a-kind Team Fortress 2 hat, hand-selected by the Joystiq staff member of your choice*.
*Choice is limited to Joystiq staff members available at time of trade. Joystiq reserves the right to refuse choice based on a variety of factors, but mostly whim.