Exclusive Locks Taken on User Tables?

Hi,

I'm seeing an "EXCLUSIVE" lock being taken on a table even though the
documentation says that "This lock mode is not automatically acquired on
user tables by any PostgreSQL command."

My SQL is
UPDATE users SET online = $1 where username = $2

username is the PK on the users table.

Other locks taken by the transaction are 1 RowExclusiveLock for the users
table and 1 RowExclusiveLock on each of the 6 explict indexes on that table
and another for the implicity users_pkey index.

The result of these locks is that concurrent calls for the same statement
are being serialized because the ExclusiveLock being requested is not being
granted.

Any thoughts on why this might be happening and what I could do to resolve
it?

Thanks,
---Marc