So you posted in Leopard ??
From a Bonjour Buddy List, AIM or Jabber one ?
In iChat 4 and 5 the app makes use of the Apple Remote Desktop Engine to iChat Screen Shares.
If you use the Finder's Go Menu > Connect to Server and use VNC://(LAN IP of other computer) then the amc will start up the OS Screen Sharing app (They have to have Screen Sharing On in System Preferences > Sharing)
I mention this as this app also uses the Apple Remote Dektop engine and may give clues as to whether the issue is there.
iChat Screen Sharing varies in that it is a VNC connection with an Audio Chat along side.
This means that whichever Buddy list (Screen Name type) you are starting from it makes two connections to the other computer.
The Audio Chat is on port 16402. The Screen Sharing part is on a Random Port.
It also means you have to be at least able to Audio Chat (Although a Video Chat is a better test bandwidth-wise.
Back to the Random Port.
In the Apple Base Station accessed with the Airport Utility in Applications/Utilities you should go to Manually Configure > Internet > NAT tab
In here Port Mapping Protocol should be enabled.
This works similarly to UPnP in other devices in that the apps tell the device which ports to open and when.
It works for Multiple computers so you should not have to set/list any Ports.
I am presuming that in both iChats you have turned On screen Sharing in the Video Menu.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb( 10.6.7)
, Mac OS X (10.6.7),
"Limit the Logs to the Bits above Binary Images." No, Seriously