Autovacuum Issues?

I'm seeing the following warning repeatedly in my postgres log. I'm
using PostgreSQL 8.4 on RHEL6.

Jan 29 04:49:57 myserver postgres[17308]: [3-1] user= db= host=
WARNING: skipping "mytable" --- only table or database owner can
vacuum it

"mytable" is owned by a non-superuser role which cannot logon named "db_owner".
Permissions for "mytable" is as follows:
db_owner=arwdDxt/db_owner
app_user=arwd/db_owner

I did not perform a manual vacuum, so this must be autovacuum kicking in.

Does autovacuum automatically use the 'postgres' role? If so, how can
I change what role autovacuum uses? One of the security requirements
I've been required to implement removes superuser privileges from
postgres and assigns those privileges to a different role. I didn't
see a config line for changing the role which performs autovacuum.

An alternative solution I've come up with is to assign ownership to
'postgres' and disable postgres from logon. However, if I do that,
this is going to require a significant modification to the system
design, as well as modification to the software, and I'm going to have
some very unhappy developers. So I'd like to avoid this route if
possible.

Thanks,

Ken