in europe we have had video calling since 3G networks were launched, and it doesn't use HSUPA. i don't know exactly how it works, but i think the video is sent like a call rather than like data over an internet connection, and will be billed by the minute rather than by the amount of data send. you ring someone on their normal phone number, you just choose an option on your phone for video call rather than normal call. the majority of 3G phones (but not all) support it, and all 3G networks i can think of in Europe support it, but few people use it as it's expensive and to be honest there isn't much of a demand - however lots of phone manufacturers add it, since in europe most people buy their phones subsidised through carrier and carriers are more likely to subsidise a phone by more if it supports features they think will make them more money, like video calling. if the 3G iphone does support video calling, it seems likely to me that it will do it through this feature of the 3G network rather than as a data connection over HSUPA, since most 3G network outside the US already support video calling in this way.
(see
http://en.wikipedia.org/wiki/Video_calling#Current_use for a picture of a 3G phone in europe making a video call, or
http://en.wikipedia.org/wiki/UMTS#Features where it talks of how the UMTS standard - ie. the version of 3G that AT&T uses but without the addiction of any HS... etc. features that speed it up - supports video conferencing)
