With this new addition to GAE, it becomes possible to 'talk' with your app using any instant messaging client (Google Talk preferred).
An App Engine application can send and receive instant messages to and from any XMPP-compatible instant messaging service, such as Google Talk. An app can send and receive chat messages, send chat invites, and request status information. Incoming XMPP messages are handled by request handlers, similar to web requests.
Some possible uses of instant messages include automated chat participants ("chat bots"), instant notifications, and chat interfaces to services. A rich client with a connection to an XMPP server (such as Google Talk) can use XMPP to interact with an App Engine application in real time, including receiving messages initiated by the app. (Note that such a client using Google Talk must use the user's password to make an XMPP connection, and cannot use a Google Accounts cookie.)