I'm using OpenDNS for my dynamic IP address, port 80 on my local reserved IP address (192.168.0.100) and using the virtual servers list (subsonic, 80, TCP, Always, 192.168.0.100, 80, 6, Allow All).
After telling Norton about the allowing access to the program and windows service it worked fine. Things did not work fine when using the subsonic hosted dynamic dns address. I had troubles accessing direct from the local network, always worked from the external dynamic ip address. Using Windows 7 64 bit through a DIR-655 A3 V1.11 (old firmware), iphone 4 client. Detailed help for subsonic is hard to find.