If you have Apple's TimeMachine backup enabled and switched to Lion you will see decreasing disk space. At first it took me a while to figure out what exactly "steals" disk space and identified it is a feature called local backup.
Allthough some documentation says it's going to cleanup once backed up, mine isn't.
If you need to disable this feature you can do so by running the following command:
sudo tmutil disablelocal
to reenable local backups:
sudo tmutil enablelocal
Please note that a reboot is necessary in both cases.