I have been using Fring for a few years on my Nokia smartphones and last fall I even made a video call via Fring from a Nokia N97 mini to a friend on Skype via his desktop. Fring had a client available for the Sprint EVO 4G just after launch (check out Joel's experiences) and I have been using it via 3G, 4G, and WiFi. Last week Fring launched a client for the iPhone that allows two-way video calling over the 3G data network, but it seems that Fring has overloading problems like we often see on AT&T and Fring had to take actions to shut off support for Skype for all Fring users.
It is great to see Fring support the iPhone 4 since it should expand the number of people I can hold video calls with, but I would rather have a working client with Skype support than more iPhone users on the network. Support for Skype was a major reason I use Fring over clients like Qik because it allows me to call family and friends who are on Skype with their laptop or desktop. Fring states this is a temporary reduction, but I haven't seen anything on when they might have servers back up to support Skype.
You can still make Fring-to-Fring calls between Symbian, iPhone, and Android owners so maybe this is a way to get more people to adopt Fring as a video call client. Have you tried using Fring on your supported smartphone?