New KB article: The _PrimarySG_ security group is repopulated every hour

A hotfix for SCOM SP1 is available for download, which solves this issue:

When the Active Directory Auto Agent Assignment feature of System Center Operations Manager 2007 is enabled, event 11470 is logged every hour on the Root Management Server. This symptom occurs because the <Server>_PrimarySG_<number> security group is repopulated when Active Directory is requeried.

KB967843 describes this issue and has a hotfix for download.