It will be really successful when you can video chat with any device whether it's an iPhone, Mac, Windows, Android, anything. And when it works over 3G.
This solves all 3 of these problems. Almost everyone I care to talk to (other than my parents) has an iPhone and will be getting the iPhone 4.
We've had 3G video calls for years. Almost all cellphones except the bottom range have it, all carriers support it, there has been massive advertising campaigns and hype about it when it was introduced, it was the whole point of switching to 3G, and yet it's a massive flop.
I'm sure you and your friends will be very happy getting it, there are always exceptions, but video calls will never surpass voice calls in mass-market popularity, they won't even come close.