Not so fast, says Blizzard. When a player asks just how far costumers might go (a live mount?), Bashiok says that there are some tweaks to the rules this year: costumes can't go over 10' in any direction, include "liquids or messy substances," or be judged on items that can't be worn or held. That would seem to disqualify the turtle mount, from official judging anyway -- they want the contest to revolve around costumes, not vehicles.
Which doesn't mean that we won't see some crazy costumes on the show floor (and obviously you'll see them here on the site, even if you're not in Anaheim). But the contest itself might be a little more low key than last year -- seems like the Sea Turtle/ATV was enough to make Blizzard dial it back a notch.