While there aren't any details as to what the DLC will be, whatever DLC is released for the upcoming Elder Scrolls V: Skyrim will be exclusive to the Xbox 360 for 30 days. After the 30-day period, the DLC will become available on PC and PS3.
Bethesda made the announcement this morning ahead of PAX Prime.
Something similar was done with Bethesda's Fallout titles and more recently with Activision's Call of Duty: Black Ops.
I really hate to see this. In exchange for helping the Xbox 360 version look more appealing, Bethesda is keeping this DLC out of gamer's hands who are purchasing Skyrim on other platforms. I know this makes Microsoft more money, and I assume that Bethesda makes some cash on this deal, but why do so at the expense of their own fans? Shame on you Bethesda.