How can I use Cron on Google App Engine?
See this post - maybe soon we'll get sort-of-cron functionality in GAE.
Here is an example http://cron-tab.appspot.com/, the python source code is available in the related project at code.google.com/p/cron-tab/
Use http://schedulerservice.appspot.com/ or http://code.google.com/p/gaeutilities/wiki/Cron
You can now run scheduled tasks with google appengine
http://code.google.com/appengine/docs/python/config/cron.html#Securing_URLs_for_Cron
According to the official AppEngine blog's public roadmap update two weeks ago, scheduled tasks (as well as background task queues) are due for the release in the moderately near future ("in the next six months," as of Feb. 6, 2009).
This page lists how to achieve Cron-like functionality on Google Appengine.