Downloadable content for The Elder Scrolls V: Skyrim, you say? I haven't even completed the main quest yet! A user on the Bethesda forums scanned through files in patch 1.5.26 and came across a few interesting tidbits that seem to tease what lies ahead for the game.
File names make reference to Snow Elves, the Snow Elf Prince, crossbows (!!!), and new feeding animations for vampires. Though I wouldn't personally take this as an end-all confirmation of DLC, that's certainly a plausible theory. Also, thanks a lot, guys -- now I'm on an endless journey through a Wiki. An Elder Scrolls Wiki, no less.