long running transactions

While doing a verbose vacuum, I'm constantly hitting things like:

DETAIL:  3606 dead row versions cannot be removed yet.

I believe this is a problem, because I still do have some empty tables
requireing up to 3-400 ms just to check if the table is empty (see
thread "slow queue-like empty table").

If pg_stat_activity.query_start actually is the start time of the
transaction, then we've gotten rid of all the real long-running
transactions.  Then again, if pg_stat_activity.query_start actually was
the start time of the transaction, the attribute would have been called
pg_stat_activity.transaction_start, right?

Is there any way to find the longest running transaction?