Rails and FastCGI are a going concern on my PowerBook.
I started with the FastCGI on OSX wiki page.
Next, I patched my Ruby-FastCGI installation. Your setup my vary, but I applied the patches to my /usr/local/lib/ruby/gems/1.8/gems/fcgi-0.8.5 directory.
Finally, I modified .htaccess in my rails application’s public directory to use dispatch.fcgi rather than dispatch.cgi. That didn’t take, as the dispatch.fci distributed in Rails version 0.11.0 references RAILS_ROOT before it has been defined. This can be fixed by upgrading to 0.11.1 or by copying line 3 from dispatch.cgi to dispatch.fcgi.
And Presto, Chango - I have a working FastCGI Rails application to work with. Sweet.