"The Always On feature allows applications to pay and keep 3 instances of their application always running, which can significantly reduce application latency. Developers can now enable Warmup Requests. By specifying a handler in an app's appengine-web.xml, App Engine will attempt to to send a Warmup Request to initialize new instances before a user interacts with it. This can reduce the latency an end-user sees for initializing your application."
This is means that JRuby (Sinatra or Rails) applications will work much better on App Engine. Users will not experience spin-up delays as they have in the past because new instances will spin-up in the background. Look for documentation on this very soon.