Configuring cron on Rackspace Cloud Sites

In the Features tab of your domain, go to "Scheduled Tasks (Cron Jobs)". Then:

1. Click on "add new task"
2. Enter a task name (such as "Drupal Cron")
3. Enter an email address for output (results of each cron run will be emailed to this address)
4. For Command Language, select "HTTP"
5. For Command to Run, enter the url of the cron.php file (for example, http://www.example.com/cron.php)
6. Select the interval to run the cron task (no more frequently than once every 5 minutes)
7. Click on "Save Task"

That should do it.