cdhowie's proxy lets you mine on port 80 without problems. In fact I'm doing just that since it means I won't have to open more than one port (I need port 80 anyway for the proxy's web admin interface). The proxy itself is port agnostic, you just need to configure the Apache it runs on to use the correct port.

If you run that proxy on Windows note that you'll have to greatly increase the Timeout and ThreadsPerChild values in the Apache httpd.conf, otherwise the proxy will give more stales than necessary and will stop working after some hours. I use Timeout 4200 and and ThreadsPerChild 1024 which works well.