Apple's iChat AV program allows you to text and/or video chat with other video enabled iChat/AIM users. The problem is, iChat AV often doesn't play nice with a very common Internet routing technology called network address translation (or 'NAT'), which is becoming ubiquitous. In such cases where your NAT router is incompatible with iChat AV, you'll often be able to do text chats, but not video.

Apple has created some tools to make it easier to test your NAT router's compatibilty with iChat AV. To test your router, just do the following:

1. Open iChat AV, click the 'Buddies' menu, and choose 'Add Buddy...'
2. Click the 'New Person' button on the bottom left, and use 'AIM' for the Account Type, and 'appleu3test01' for the Account Name, then click the 'Add' button.
3. Back in your buddy list, you should see a buddy called 'appleu3test01' - click on the green camera icon to the right of their user name to start a video chat test. If the camera is faded green, try waiting a few minutes, or try using 'appleu3test02' or 'appleu3test03' instead of 'appleu3test01' as the account name, in step 2, above.
4. If an Apple commercial starts playing, your test was successful! If you get an error, then your router is probably not compatible with iChat AV. This can sometimes be worked around by changing router port forwarding settings, but this will only work for one Mac behind any given firewall).
5. Apple has published a list of officially compatible routers here. Additionally, I've personally discovered the following routers to work or not work:
   
   Compatible:
   • Linksys WRTP54G (version 1.00.62)
   • Netopia 3346N-002 (version 7.5.1r2)
   
   Incompatible:
   • Netgear ProSafe FVS338 VPN firewall