It's available now from Sparkfun! $59.95.
And cool home page promo on the Sparkfun site here.
Here's a quick FAQ for people just now coming to the site, or hearing about ArduPilot Mega for the first time:
We recommend this board just for people who want to beta test or tinker. For regular users who want a plug-and-play (ish) experience, it's best to wait a few weeks for us to release the IMU shield, code and documentation. (The Sparkfun board will also be tweaked a bit, with mostly cosmetic changes to make some pin labeling clearer, in the next batch.)