I am not a fanboi
just wanted to get that out of the way before i start.
Yes, many phones have been 'able' to do video calls for some time, my old N97 was one of them, but there are many reasons why it didnt take off.
People keep saying "only over WiFi..." i think this is an admission from Apple that they have built something that not all of the networks can handle, look how much trouble AT&T and to a lesser extent o2 had with the massive uptake of 3G use when the 2nd gen iphone was released. As such, buy having video calling restricted to wifi, it takes the burden off the cellco's meaning that other (more important?) services, such as data and voice calls won't suffer as a result.
Que the, "but all other video mobiles use 3G, so why cant the iPhone". I'll give you my opinion in 2 parts.
1) Because currently so few people use video calling that its not caused problems over 3G, you wait till millions of Iphone users wanna do likewise.
2) The quality of current video calling is abysmal, where as I assume Apple are of the opinion that 'if your going to do something its worth doing right' and because of that, the quality 'should' be significantly better, which would in turn, put a massive strain on the cellco's, which would not be present over WiFi.