With 8.1, autovacuum is integrated into the main backend, it's no longer a external contrib module. This has several benifits. First, it's easier to setup, just uncoment the setting int he postgresql.conf. Also, you can now set vacuum and analyze thresholds as well as vacuum delay settings on a per table basis. This was probably the biggest problem people had with the contrib version. In addition it is more reliable since DB activity data is no longer lost between database restarts. Also, you can now exclude specific tables from being touched by autovacuum.