Ooooh yeah, the time has come. After applying a new patch to the game on Xbox Live (not sure on PSN,) which fixes some issues with the game (wrong numbers shown on the mission select, levels with bad lag like Adabat Day Act 1 and Empire City Hub getting fixed to look a bit better, and the "lost" area of Adabat Night now getting a static water texture to show where the water level is,) downloadable content has come to Sonic Unleashed.
The first pack, now available on Live and PSN, is titled the Chun-nan Adventure Pack and adds four days stages and two night stages to the game's roster. These stages are smaller ones and add things like a hard mode of the levels along with some other things (I can't give good descriptions since I've not played them yet; they're currently downloading.) However, I am told that the werehog level now explores the "lost" area of Chun-nan Night. This area was used in the game in two missions, but now it's used as an alternate goal ring location. This means the other "unused" areas may soon lose that status and be explorable without walk through walls glitching.
The file size is about 500MB, so these are definitely new stages and not keys to data on the disc.