This is a really nice idea. In essence:
Pictorical are a publishing house, who sell ebooks for iPad. You apply to register with them as an artist and --provided they like your illustration style-- they sign you up and you choose from their list of available books and illustrate it. When it’s done, they sell it as an eBook and you [as the artist] get half the profits.
Applications are closed at the moment, but it might be one to keep an eye on for the future.