I’ve not seen an official annoucement yet, but this Google Groups post by Pamela Fox indicates that the ads layer has been released.
This layer will allow you to put AdSense inside of your Google Maps that you generate via the API. The ads will be for things like hotels, with your placemarks all around in their normal locations. Here is a sample of what an ad looks like:
It’ll be interesting to see how this works out. I’ll have to try it out on some sites of mine and see what it does.