You can get the details here, on our main webpage.
The short version: The main game-derived costumes are eligible for a limited edition Vorpal Blade as the grand prize. The Army of Alice contest is a random drawing from all of the Alices, and those winners (yes, plural) get some awesome Alice-themed goodies.
I'll be posting here more, if you'd like to be kept in the loop. There's all kinds of very neat things coming up.
(You're in our Alternate Reality Game, right...?)