As more people are upgrading to Leopard it is becoming apparent of some issues that are arising between Leopard and Tiger machines using video iChat. Some of the problems are as a result of the modem/ routers being used as opposed to the operating system itself. In particular there are some issues with some of the D-Link products.
Here is a rough guide to what is compatible (as far as I am aware anyway):
Tiger to Tiger = Works
Leopard to Leopard = Works
Tiger to Leopard (LAN) = Works inmost instances
Tiger to Leopard (internet)= Works sometimes (Does not work with some D-Link products)
It is possible to fix the D-Link issue between Tiger/ Leopard however:
a) it requires upgrading the D-Link firmware and altering the modem/ router settings
b) most Tiger machines may be upgraded to Leopard soon (if you are not comfortable upgrading firmware or messing with your router, upgrading the Mac OS to Leopard may make for a simpler fix in the long run)
There are possibly (and probably) compatibility issues with other manufacturers, however this is what I have experienced to date.
How to fix the D-Link problem:
You need to open up ports 16393 - 16402 on the firewall. Once this has happened, all should be good!