I'm not really sure! I was invited into the program through the partner channel at Google because I was working for a company that had already done a lot of development on the app engine platform. I was one of the early adopters of GAE, as I started using it the first year it came out. One of the apps I co-wrote became a case study Google used.
See this is kind of an example of how Google is awesome at engineering but short on communication. I can't even explain how to get into their fold, so to speak.