So this morning, I decided to try to get to the bottom of what causes iChat AV to fail with my router. I was testing with my stepfather who also has a WRT54GS router (by Linksys).
I read on the Apple site that some routers break when using UPnP, but in general, they have a list of ports to forward. Since I’m planning on multiple Macs in the house, forwarding ports to a single system isn’t going to work for me. I disabled UPnP, then used a remote assistance session on my PC to do the same on my stepfather’s router. Then I returned to the Mac, fired up iChat, and bang, what do you know… that resolved it… without forwarding any ports. I can now have an iChat session with my Mac to my stepfather’s PC without issue.
So, here’s the tip: if you have a Linksys router, get rid of UPnP. Honestly, it has security problems anyway.
Disclaimer: this is with the latest firmware revision for the router. For my router, this was v4.71.1.