A brief update on the App Engine roadmap, then...
JRuby works pretty well on App Engine, but can be slow to spin up and has a higher cost in CPU and processing time. Dubious rescues you from having to write Java servlets when you need more performance or better spin up times. Using the strengths of the Mirah language, it lets you write code with the performance of Java, that looks a lot like Ruby.
And finally, Google is building a new generation of APIs, with a new approach to clients that will have a significant effect on how the Ruby community interacts with Google services.