Yahoo's getting in the game. REST based APIs for Search, Images, News, etc.
Yahoo's differentiators:
I think another difference is that Yahoo knows how to support a community launch better than Google. Jeremy Zawodny alone will do more to promote the APIs than the Google AdWords API team has time for. E.g. a quick tour of the sample code shows more depth than the Google AdWords API samples. A nice perl example, even a JavaScript sample. PHP is kind of weak, however...
Paul Bausch, a creator of this fine blogging service (blogger), and author of Amazon Hacks, has a quick Yahoo API sample up on O'Reilly site.
BTW, if you want to get started with the Overture APIs, they are there, you just have to look carefully. Unfortunately, you need to process DTC-XML for Overture. So once people learn SOAP, REST and REST / DTC-XML, we can expect some cool apps!