I have Snow on a Macbook Pro and can Video to Leopard users.
In System Preferences > Security an I have the Firewall ON and iChat in the List in the Advanced Button (Automatically allow Signed Software...)
Stealth is not set as iChat sends a Ping and stealth would block this.
I would also check your routing device.
If this is a fresh Install of Snow Leopard you may not have the same IP from the routing device and any Port Forwarding you were using may no longer be pointing at the computer.
In System Preferences > Network > Advanced Button > DNS tab make sure you make has DNS Servers listed.
Are you getting any sort of Pop up to send to Apple on the chat failure ?
(you don't always get it on the Declined message).
8:46 PM Sunday; August 30, 2009
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"