I had this same issue and it would happen about once a week. Before my 30 days was up, I exchanged it for a new one. The new one has done it I think a couple times in the two weeks I've had it. There are a few other threads about this topic out on the bulletin boards, and there really isn't a good answer for it which is frustrating. I guess this is just a nice "feature" of the Droid.
A few recommendations is remove all the extra applications as you might have an app that is causing it to crash. But what is the point of having a smartphone if you can't download applications, might as well go back to a freebie phone if that is the cause.