Fill rates on iAd, to date, have been pretty poor. It will be interesting if they can do any better with audio ads. Unlike Pandora, which is only available in the US, Apple has to sign deals around the world for their service.
In fact, anything involving ads and Apple is interesting because it is a new area for them. What other Apple product is supported by advertising? In the past, Apple has presented itself as a company that shuns advertising. For example, at WWDC 2011, Jobs stressed that iCloud Mail would be ad-free.