Stuff…

OS: Linux
CP: Plesk 9.x

Where does Plesk 9.x store backups generated from the scheduled backups initiated via the control panel.

/var/lib/psa/dump stores a lot of backups including the psa mysql database backup (incase you, or Plesk, corrupt the psa database).  Client backups can be found in: /var/lib/psa/dump/clients

The backups of the vhosts/www data appears to be in tar.gz (tgz) files so you can easily extract data using tar.