The Elder Scrolls V: Skyrim arrives in stores this November (likely by way of carrier dragon), it'll assuredly feature many, many hours of content. Nevertheless, gamers will want more, and Bethesda has promised post-launch content to be of the "substantial" variety. The first two pieces of that content will arrive exclusively on Xbox Live Marketplace at least "30 days before it's available anywhere else."
Bethesda revealed the news this morning via press release, which also indicated we'll see more than two "add-on content drops" for Skyrim after the game launches. Given the kind of expansions we've seen for the Fallout titles and Oblivion, we expect the folks at Bethesda have some serious plans in mind for DLC support.