If you look carefully at the Google I/O 2009 sessions page, you’ll see this session listed:
Offline processing on App Engine: a look ahead
App Engine was designed to run request-driven web applications, although this will change in the coming year with the release of a number of offline computing components. In this session, we’ll explore the task queue/executor model of computation and some of the more interesting applications.
While I am under NDA with the App Engine team for some things, this was quite an unexpected thing to look forward to!
In other words: Games, Instant messaging, and Micro messaging types of applications will be possible with App Engine sometime during this year (which is not to say necessarily launching at Google I/O). 2009′s going to be very interesting.