BioWare has a dedicated team working on DLC. Why wasn't it included in the main game? Because the content wasn't nearly complete by the time the main game was locked and into QA, certification and manufacturing. The main game and Warden's Keep are in different stages of development and there is no way that WK could be released with the main game at this stage.
This is the bit that bothers me the most. I mean, I don't have a problem with content being available day one that wasn't able to be finished in time to be included on the disc. But then, that entire situation/explanation suggests that if it could've been finished in time it would've been included on the disc and thus (essentially) free. I just don't understand how you go from that to releasing it day one (or day really-low-number) for $5-$10.
I don't mind the free-with-new-game DLC much, but I am a little skeptical that it became DLC just because it wasn't ready in time.
However, the game itself sounds great and I hope this doesn't detract from that too much.
Also what's up with the marketing for this and ME2?