We recently came across an issue when migrating a Plesk 9 system onto a new Plesk 11 server with scheduled tasks and the wget process.
One of our customer domains had a scheduled task similar to: wget -q -O /dev/null "https://www.domain.com/path/to/webpage/to/run/" and the new Plesk 11 server generated an email error report stating: -: wget: command not found.
Plesk 10 & 11 restrict execution of scripts via cron task scheduler. To resolve this issue check out page 166 on parallels admin guide: http://download1.parallels.com/Plesk/PP10/10.2.0/Doc/en-US/pdf/plesk-10-administrator-guide.pdf
Alternatively run the commands via root within Plesk.