I just setup my first mongrel server behind a apache proxy. I recognized
that a lot of explanation in the wiki and on other external pages
configure mod_proxy to connect to a localhost but simple do not
configure mongrel to listen only on localhost as well. This leads easily
to security issues for unexperienced administrators.
sample doc:

My proposal is that for the mod_proxy examples the listening IP is
always added. This might encourage to use a local port as default

Best Regards

Sebastian Hennebrueder