Those of us that chose to jump right in and jailbreak our phones have had VoIP calling over 3G for quite a while using VoIPover3G available in the Cydia store. Other users who were not so daring were only able to use VoIP apps when connected via WiFi. But I must say, in my area attempting to use Skype over 3G sucks! You have to have a really, really strong signal to get a good connection or talking is very choppy and it lags. I am hoping that since they have made changes on the network to allow for this, that it will also allow for nice strong connections.
I'm certain that Skype will be updated at any moment to allow the change, with others following close behind. And with Google's recent purchase of Gizmo5, the natural progression should be a full VoIP service from Google soon using Google Voice. I will certainly be waiting for that.