Steam will have micro transactions

In the near future it looks like Steam will have micro transactions.

Want to offer your customers a scabbard for their sword? New tires? The latest in night vision goggles? Now you can through microtransaction support in steam works. Your item server can now be easily integrated with Steamworks so that Steam users can buy and instantly use the items they want – all without leaving the game. Steam handles all of the billing through the user’s steam account.

Source: SteamworksBrochure2010.pdf