Shutdown fails with both 'fast' and 'immediate'

I develop an app that uses a back-end Postgres database, currently 8.3.9.
The database is started when the app starts up, and stopped when it shuts
down. Shutdown uses pg_ctl with -m fast, and waits two minutes for the
process to complete. If it doesn't, it tries -m immediate, and waits two
more minutes before logging an error and giving up.

One user, on OSX 10.5.8, has a script that stops the app each morning, to
upgrade to the newest build. In his case, both the fast and immediate
shutdowns time out, and Postgres continues running for at least 2-4 hours.
At that point he brings up the terminal to kill all the back-ends manually,
so we haven't seen it finish shutting down on its own yet. It is in fact
shutting down, because all queries fail with the 'database system is
shutting down' error.

The query running during this time is a DELETE that runs as part of the
application's daily maintenance. The size of the DELETE varies, and in his
case happened to be unusually large one day, which is apparently what
triggered the problem. Since the DELETE never gets a chance to finish, the
problem recurs every morning.

I'll obviously need to deal with that query, but I'm concerned that Postgres
is unable to interrupt it. Why might this be happening? Thanks,

David