The Avatar: The Last Airbender Big Bang is signing up writers for this year's round as well as taking story prompts. The deadline is May 31st. I highly recommend joining it. It's been very well run in the past.
I'm personally on the fence about signing up. I'd certainly like to, but I've withdrawn from every other fandom activity (including a generous pinch-hitting opportunity for the Rare/Crack Pairing Exchange by that community's moderator) so I can refocus on my existing projects. 2010, frankly, sucked for me meeting deadlines, and I need to salvage 2011. In addition to completing the next chapter of Full Metal Bitch before I've lapsed a whole year's worth of updates, I have two Help_Japan projects I'm currently writing. Plus I'd like to take a break from AtLA and do something in another fandom once my current slate of projects is finished; maybe Sailor Moon.