All I know is on the Bethblog, it only states the 360 and Dec. 4th.
However... the game is ported from the 360. So we might have to wait til they are done porting it to PC. (Sorry PS3 players.. it only makes sense for them to give you DG first before Dragonborn..)
The game is ported from the PC in both cases actually. It's just far easier to port to the 360 from the PC. And vast majority of that is engine-level stuff anyway. In the case of plugins, the 360 and PC version are generally pretty much the same.
Personally, not expecting PC on Day-1, it's exclusive, if it ends up not being the case, well then I get to be pleasantly surprised. It's not like the change that happened when FONV DLC was no longer timed exclusive.
DLC simply means a downloadable addon to the game, no matter the size. Many games have DLC that add like 3 items, that's it. You're thinking more 'Expansion' than DLC.
And technically the first DLC released for Skyrim was [and still is] PC Exclusive, "Skyrim High Resolution Texture Pack". Sure it was free, and only really consists of some buggy textures, but it was technically DLC.