Notice anything? How about all of that Dungeon Siege 3 DLC? It was discovered by the fine folks over at RPGSite, though they were admittedly looking for something else: Final Fantasy VII and VIII. Apparently both of those games are on the Steam servers, but unable to be purchased. This Content Description Record is updated alongside Steam and reportedly lists everything on Valve's servers.
All of the listed DLC were pre-order bonuses: Burning Band of Scorch and Talisman of the Grand Mage were GameStop exclusives; Bite of the Arakun was an Amazon exclusive; Sacred Heart of the Legion was a Best Buy exclusive; Dell offered the Annulus of Force, and AAFES offered the Medallion of St. Elys. It would appear all of this DLC will be offered at some point on Steam.