http://serendipity.ruwenzori.net/index.php/2007/08/06/mdadm-rebuild20-event-detected-on-md-device
Ever get a little scared reading the logs... Well, it would seem that a bit of searching is in order sometimes before assuming the worst.
mdadm: RebuildStarted event detected on md device/dev/md0
The logs are just saying that Debian likes to do thing for you, check your cron jobs!