600 words or less. Your story gets read on the air. That's your main prize, actually. Published on their site, too, though. They just released the theme for the new contest, due 1/23/11, this weekend! Information here.
I thought the winner from the contest that ended in December was pretty great. It was called Roosts.
I've been interested in flash fiction, stuff at the 1000-words-or-so length, lately. It's amazing what folks can sometimes do with very few words.