I think for NaNoWriMo I'm going to go with my idea from the submission to Permuted Press' The World is Dead anthology. In my rejection letter, the editor liked the idea but passed on it due to some predictability with the ending. In hindsight, it was fairly obvious what was going to happen, but I think the world has a lot of potential. That short story was about "the past" and my NaNoWriMo project will take place in "the present" of this world.
I spent some time yesterday jotting down a few notes. Normally, I start a new idea with very little outlining ahead of time, hit about chapter 6, and then take a break from writing to do some seriously planning. Since I'm not going to have the luxury to stop in the middle of this piece to iron out the rest of the plot, I'm trying to get as much of a general flow of events as my brain with allow. Besides, I can stop at 50,000 words to reconsider everything, right?