I switched from WEBrick to Phussion Passenger after this guide: http://developer.apple.com/library/mac/#featuredarticles/PhusionRails/index.html. I made use of PassengerPane to configure it.

Now I'm able to access my application at myapp.local, rather than localhost:3000

However, I'm not sure how you can can get on in the outdoors. It was once ip:3000

My vhost.conf file appears like this:

<VirtualHost *:80>
  ServerName myapp.local
  DocumentRoot "/Users/martin/myapp/public"
  RackEnv development
  <Directory "/Users/martin/myapp/public">
    Order allow,deny
    Allow from all
  </Directory>
</VirtualHost>

You have to add ServerAlias YOURIP:

<VirtualHost *:80>
  ServerName myapp.local
  ServerAlias YOURIP
  DocumentRoot "/Users/martin/myapp/public"
  RackEnv development
  <Directory "/Users/martin/myapp/public">
    Order allow,deny
    Allow from all
  </Directory>
</VirtualHost>

You can include as numerous aliases as you would like (with real domains for instance).