It looks like you no longer need to have a WiFi connection to make Skype calls on your iPhone. A change to the iPhone SDK is now allowing Skype calls over 3G. This means that you can make voip and video calls as long as you have good reception with your iPhone. Users don't have to do anything special as the change took place on Apple's end.

There's no official app from Skype, but you can use Fring to make the calls. Very cool!