Query stucked in pg_stat_activity

Hi all,

I have a postgres system where we just migrated a fairly big data set.
The application accessing it is a cluster of servers which do burst-like
processing, i.e. when they have some work to do, it will be distributed
in the cluster and the data base will be under fairly high load.
On our first test run everything went fine, the only strange thing is a
row in the pg_stat_activity, which has a row about a query which is long
gone, the process pointed by the procpid field is not existing.
I've executed a "select pg_stat_reset();" as superuser, and all went
away except the offending row... I wonder what can cause this behavior,
and how to get rid of it ? I'm using the pg_stat_activity view to detect
long running queries, and I do expect a few more to pop up in time...

Thanks,
Csaba.