Deadlock on "select ... for update"?

Several times recently one of our databases has gotten stuck with the
following situation:

postgres=# select datname, procpid, usename, current_query from
pg_stat_activity where current_query != '<IDLE>';
datname | procpid | usename |
current_query
------------+---------+----------+--------------------------------------------------------------------------------------------------------
emolecules | 13503 | customerdb | select tableid from hitlist_table_pool
where hitlistid <= 0 for update
emolecules | 32082 | customerdb | select tableid from hitlist_table_pool
where hitlistid <= 0 for update
emolecules | 17974 | customerdb | select tableid from hitlist_table_pool
where hitlistid <= 0 for update
emolecules | 31299 | customerdb | select tableid from hitlist_table_pool
where hitlistid = 0 limit 1 for update
emolecules | 30247 | customerdb | select tableid from hitlist_table_pool
where hitlistid = 0 limit 1 for update
postgres | 1705 | postgres | select datname, procpid, usename,
current_query from pg_stat_activity where current_query != '<IDLE>';
emolecules | 28866 | customerdb | <IDLE> in transaction
emolecules | 21394 | customerdb | select tableid from hitlist_table_pool
where hitlistid = 0 limit 1 for update
emolecules | 22237 | customerdb | select tableid from hitlist_table_pool
where hitlistid = 0 limit 1 for update
(9 rows)

It's obvious that they're all waiting ... but for what? The "<IDLE>"
process looks like the culprit, but how do I figure out what it's doing?

The next time this happens, what queries can I run to help diagnose what's
going on?

This is PG 8.4.4 on Ubuntu 10.

Thanks,
Craig