i have the same problem too. local and external wp-cron.php calls don't work.
i have an entry in my error.log file:
[Thu Aug 08 14:50:00 2013] [error] [client ****] check_postie
but nothing happens. i always have to start postie manually. perhaps it has something todo with the latest plesk update to version 11.5. after this update the connection from localhost to the hosted websites is broken. so i switched to an external cron job, which in general works (according to the log file entry.
but it doesn't trigger the mail fetching properly.
