This is an issue I had with iChat and my Netgear WTG624 v3 and it took me a while to understand how to fix it.
First, let me give you an overview of the problem. What happened was I got my Netgear router, hooked it up and started using it and everything seemed fined. Well, my wife works from home occasionally and connects to her office via VPN. She went to try this and it wouldn't connect so I started looking around the router settings. I finally decided it was because the SPI firewall was disabled (I don't know the actual reason, but enabling this seemed to fix it). Then I noticed that once this was enabled, my iChat wouldn't connect to the AIM server. All other protocols were working within iChat, but AIM wasn't. I then tried a different mac AIM client and that seemed to work correctly. I was also able to connect to AIM using Trillian on my pc so obviously it was something with iChat. Well, I googled around for a bit and didn't come up with much so for a few months we just dealt with enabling / disabling the SPI firewall within the router everytime she wanted to connect to her VPN.
I finally got sick of doing this so I started looking around. It turns out that all you need to do is change your port in your iChat settings. Here are the specific instructions I found at Appleinsider.
Hopefully this helps someone else.. I know it was a pain in my ass for a while.