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From this page:

"You did not respond error message: What? Of course I responded... I clicked the Accept button. Then why does iChat give an error message stating that I did not respond? Simple answer: firewall ports. You most likely got this error message when trying to video conference with a PC user who did not go through all the steps listed in the XP internal firewall settings sections. Tell them that turning off the internal firewall is often not enough... go through the entire process listed for best results. If you then still get this error message start looking at other places (your router, ISP or corporate network) that could be blocking the ports listed below."

There's also an invaluable iChat Help/Advice Repository here.
 
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