Thanks everyone. It appears that we had hacked the 502.pgsql script for our 8.1 build to disable the daily vacuum. I was not aware of this when building and upgrading to 8.2.
So it looks like for the past two weeks, that 36 hour db-wide vacuum has been running every 24 hours. Good for it for being reasonably non-intrusive and going unnoticed until now. :)
Although apparently not related anymore, I still think it was a good move to change autovacuum_freeze_max_age from 200 million to 2 billion.
Steve