You also need correspoding rules in ipf.conf to actually allow this traffic
to pass through.
RDR is a two-phase process. Once you get it to work, you'll wonder how you
could have ever done without it.
It's one of the coolest freatures in IPFilter.
I do not believe it matters that port 80 is a privileged port; what does
matter is under which user the process listening on port 80 listens -- and
that user in your case should be "httpd" -- just a regular user with zilch
of extra privileges.