I have written a few things in Oracle's pl/sql, but never in postgres. Remember, this gives the user quite a bit more access, so I would keep an eye on what (s)he does.
After a quick google, it looks like this might explain what you are looking for:
http://www.faqs.org/docs/ppbook/c19610.htmI can not attest to how valid it is, as I have never had to add permissions for pl/pgsql access.
Let us know how it works out.
-James