Games would still require a computer to run so unless we're missing something this presumably involves some kind of external box hooked up to an HDTV to stream the Steam signal from a PC.
There are currently over 30 million Steam accounts worldwide and this expansion is designed to reach more even mainstream gaming audiences. "Our partners and customers have asked us to make Steam available in more places. With the introduction of Steam on the Mac, and soon in Portal 2 on the PS3, we've done just that," said Valve's VP of Marketing Doug Lombardi. "With big picture mode, gaming opportunities for Steam partners and customers become possible via PCs and Macs on any TV or computer display in the house."
Valve is also planning to share information it gathered during its launch of Steam Microtransactions late last year, which lets developers create an in-game economy using the system within the Steamworks Software Development Kit.