"canceling autovacuum time"

I'm seeing "GMTERROR: canceling autovacuum task" lines in my logs.

2012-02-27 23:53:28 GMTLOG:  checkpoint starting: time
>2012-02-27 23:53:31 GMTERROR:  canceling autovacuum task
>2012-02-27 23:53:31 GMTCONTEXT:  automatic vacuum of table "<somedb>.pg_toast.pg_toast_33254"
>2012-02-27 23:53:32 GMTERROR:  canceling autovacuum task
>2012-02-27 23:53:32 GMTCONTEXT:  automatic vacuum of table "<somedb>.pg_toast.pg_toast_33485"
>2012-02-27 23:54:29 GMTLOG:  automatic vacuum of table "<somedb>.pg_toast.pg_toast_33254": index scans: 1

It was suggested to me that perhaps I had an exclusive lock on the table that was being vacuumed (e.g. that I was running some DDL that was conflicting with the autovacuum). I'm quite certain that I'm not running any DDL at the time this happens, all i'm running are:
* SELECT ... FOR UPDATE
* UPDATE
* INSERT
* DELETE
Does anyone have a suggestions as to what the cause of this error might be?  

I'm using Postgres 9.0.

Thanks.