"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.
Nevertheless, it's encouraging to see Google officially support what many consider an essential tool in running modern web applications. It's also helpful to have cron integrated with the GAE dashboard, so I can see everything scheduled to run and the result of the last run.
The release came at the perfect time for me, the day after I launched an app that requires regular update tasks to run in the background. It was a boon to simply add a yaml file to my app and not have to hack the functionality in.
We've implemented cron on GAE by using silent ajax calls from clients to kick start things. Works pretty well ... as long as at least one page is open in a browser somewhere.
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).