Looks like he's in it to win it. Technically, he could still withdraw but apparently it gets a lot more complicated--court order required.
My favorite news of the day is that an hour before the deadline, Mitt the Twit put on his clown shoes and officially changed his position from "Er, um, I don't agree with Akin" to "I think he should step down." No word yet on whether the McCaskill campaign sent Willard some flowers as a thank-you for writing their ads for them.