"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 have a cron job set to run once a minute (which I am impressed they allow).
Also, I am currently using the remote-api to create tasks that I run on my laptop or another server. I imagine the offline processing is pretty similar.
That is indeed pretty stupid, but on the other hand you can set up a task that runs often and then split the time consuming task into several smaller tasks and then only execute on task at a time.
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).