psql hanging

What would cause psql to hang indefinitely when the backend disappears?

We have a script that uses psql to insert a record (TCP connection to DB
on different machine). The command is basically
psql <connection_stuff> -c "insert into..."

A while back I had to restart the server and today discovered that some
of the client machines have psql processes dating back several months.

Obviously no TCP connection on the server end but client-side shows the
connection as "ESTABLISHED".

-Steve