I'll be honest, I don't "get" the concept of pickup games for D&D. It makes sense for no-story-necessary, short term stuff like Halo levels or a quick spin around GT racetrack (or sports in the park), but I just don't see it for D&D.
Of course, D&D means different things to different people, but if all you're looking for is a sit down to bash some Orcs I really think WoW (or Diablo 2, or whatever) is better suited to what you're looking for. That's not a slam, because I love my Starcraft or AoE lan parties, it's just a "Are you sure you're using the best tool for the job?" question. To "reduce" D&D to playing a couple Diablo levels seems like you're missing the best parts of the game (the RP part).
IMO, of course.
On the other hand, of course, DDI may end up being the cat's pajamas for groups that are spread out geographically. But weekly regular play, which continues a story and character development, is quite different from "pickup games."