We have an Apache document up that you could follow:
And, then you would either create a new virtual host for your blog (like
blog.rails.com) or you can use the --prefix option for mongrel and put
the site at /blog. The virtual host is what rails-core does, so that's
probably what you should do.
After that it's just a matter of setting up the second mongrel/rails
configuration to run your typo install.