My main/base/pgsql_tmp directory has over 5.7 millions files

Is there any reason why these files aren't cleaned-up ? We have just
installed postgresql-9.1.3 and it seems we have this problem since this
update.
I haven't checked the ratio (ls takes more than 5 minutes to return) but
most of these files seems to be empty. It causes me problem when I try to
tar/untar my data directory to have SR on a slave machine.

Would it be safe to have a cron script cleaining up these files once a day
? Better yet, any idea why those files are not automatically deleted when
they are no longer needed ?

Francois L.
Montreal, Canada