A SDK will be available after LR has been released -- quite how long after, I don't know. No idea what model it will incorporate, although it would be nice if it's consistent with CS2's scripting.
Modular design + SDK sounds very promising. It'll be interesting to see what extensions people come up with. I relish the prospect of being able to make small tweaks to LR to shape it to my way of working with files, plugins, etc.