"A cron job will invoke a URL at a given time of day. A URL invoked by cron is subject to the same limits and quotas as a normal HTTP request, including the request time limit."

So you can't do cron tasks which last more than 30 seconds. Too bad.

I'm looking foward to "Offline processing on App Engine: a look ahead" presentation during the Google I/O (http://code.google.com/intl/en-US/events/io/sessions.html).