I already found the answer, sorry... my mistake for not checking the
8.2.0 release note first. It turns out that such behaviour is expected
in 8.2.0, i.e. autovacuum process no longer add an entry to the server
log.
http://www.postgresql.org/docs/current/static/release-8-2.html
But then, another question arises... how could I know whether it's
working if it does not add any entry to the log?
an excerpt from the above link: "pg_stat_activity now shows autovacuum
activity."
I expected to see a new process showing up in pgAdmin's Server Status
window while autovacuum is running, but there was none.