As I wrote last April you can use Objectify in Java apps to reduce loading request times (avoiding JDO overhead) but running JRuby and Clojure applications on AppEngine has been hindered by still long loading times. The new feature forthcoming to AppEngine (SDK available, but server side support is not in place yet) of allowing paid for apps to keep three compute instances always running will open up the AppEngine platform for other languages and frameworks. On the development side I have had good JRuby and Sinatra experiences with the AppEngine SDK but have never wanted to deploy anything other than experiments - that will change now.