Wow, this is an old thread indeed.
The phantom old box is in the monitoring database. I found it with SQL Management Studio, but couldn't remove it or edit it so that the error would have gone away.
My current theory is that if you delete the old server computer object from ADUC after demoting it, it will stay as a ghost in the monitoring database. Maybe deleting it from the SBS console would remove it from the monitoring as well?
I'm going to do a migration next week, I'll try if this is the case.