Was banging my head with this trying to figure it out a few times and figured I'd post the solution here.
Due to googlebot and the load it was generating on my server at the colo, I had to move svn.annvix.org from my primary webserver to a spare box on my cable line at home. The problem was if I referred to http://svn.annvix.org/ from home, I was never able to connect to it (browser would return unable to connect), so I figured I needed a way to tell shorewall to redirect anything coming from the local LAN to the firewall to be redirected back through the firewall to the specified machine on the local LAN again. Sounds like it should be pretty straightforward but it wasn't until I found this entry in the shorewall FAQ:
(FAQ 2) I port forward www requests to www.mydomain.com (IP 22.214.171.124) to system 192.168.1.5 in my local network. External clients can browse http://www.mydomain.com but internal clients can't.
Following the directions there solved the issue for me.