Fuzz,
It sounds like you've taken most of the correct steps.
Just a few questions...
did you forward those ports as UDP or TCP?
Most of not all of them should be UDP I believe. However, if you are on the same LAN it shouldn't make a different at all. Try using bonjor messaging when you are on the same LAN, see if that helps. You can enable it from iChat's preferences.

I've had problems with iChat traversing NAT routing, but I've never had a problem when both parties are on the same lan.

Also, drop me a PM if you need someone to test with you...I can point my camera out the window and set it to auto accept chats and you can try all you like.