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You will be able to video chat with someone on a Mac, but not via iChat. They will have to have Facetime app installed on their device as well.
 
Will it be possible to use Facetime on an iPad2 to speak with someone on a Mac using iChat?

I don't think so since there's a separate FaceTime app on the Mac.
I wish FaceTime was built into iChat and that iChat was on iOS devices. Most times I start out text chatting with friends/family and then switch over to webcam chats for a little while. I hardly ever webcam someone right off the bat.
 
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