Not being able to use Skype on a 3G cellular connection on your iPhone may not be such a big limitation. AT&T's cellular coverage blows so badly that it may be easier to find a wireless hot spot anyway.

I've heard folks rag on AT&T's coverage quite often, and it was the one thing that made me reluctant to buy the iPhone and give up Verizon. But I have to say I haven't had any signal issues since I made the leap. Though i don't doubt that the coverage is sucky for a lot of people; At the end of the day Verizon's service is much more consistent. I guess it depends largely on where you live.

I've had good results using Skype over Sprint's 3G network with HTC Titan. The common consensus is that Sprint's network is crap, but I've never really noticed many issues... even while driving cross-country and tethering in the middle of the great plains and utah deserts. Maybe this is because Sprint cross-roams with Verizon?

Anyway, using something like Microsoft Portrait, where as well as VOIP you get cameraphone video streaming for that Dick Tracy watch effect, the 3G network breaks down. It just doesn't work without lots of chop. For video, it seems you have to go to WiFi. Maybe future 4G networks will be able to handle video?