This year I'm actually leaning quite heavily towards participating. I am smack in the middle of my WIP (having just crossed the 50,000 word milestone last week) but this time around I actually have a real use for NaNoWriMo (other than the coolness of knowing I've written 50,000 words in a single month).
I have to develop the details of a specific back-story for my WIP. One of my POV characters depends heavily upon this. So, taking November off to write this back-story will provide a huge benefit to my WIP when I return to it in December. On the downside it will almost certainly guarantee I won't finish the first draft of the novel before the end of the year. On the upside when I do finish it in early 2009 it will be all the better for having written the back-story.
So, I think I'm going to give it a go. Already my mind is formulating the story, popping up with little tidbits. I've opened up a notes doc and jotted those down, and I think I know which scene to open this with. I have 2 weeks to ready myself for a month of marathon writing sessions. By then I should have a clear idea how to present the story. I'm not much of an outliner, but I did jot down the major plot points for this work.
This should be interesting. Who knows, maybe someday I can turn the NaNoWriMo draft into a novella or even expand it into a full fledged novel later on. I do know I need to develop this story in order for my current WIP to feel right, in order for me to know exactly how to portray one of the major plots of the WIP. Also, seeing as this back-story is the catalyst for all of the events in my current WIP it will help out in other areas in this story as well. And knowing how I write (and that during the actual writing process I will invariably come up with new cool concepts or better ways to describe some existing ones) that alone will make NaNoWriMo worth every moment.