Apple has just released iAd Producer, a Mac program that allows developers (and non-developers alike) to create iAds using a simple point-and-click interface. The application has a variety of templates to choose from, along with built-in animations and components. The ads can then be simulated on a Mac or iOS device.
At the moment, the iAds program is only available to select companies, but the release of this program hints at a public release.
Note: You must be a member of the iOS Developer Program to download iAd Producer.