In the hoopla surrounding the launch of Skype for iPhone, the powers that be noted that the application would only allow voice calls over a Wi-Fi connection. 9-to-5 Mac reports that in actuality, Skype for iPhone will allow you to make voice calls over the AT&T 3G network too. I'm not sure how it's going to last though.
9-to-5 Mac reports that the sound quality (over 3G) was as good, if not better than normal AT&T.
It should also be noted that Skype, unlike Apple's phone and text applications, doesn't run in the background:
The Skype application must be started and signed in to receive incoming Skype calls and chats. If the application has been closed, incoming Skype calls cannot be received and chats will not be delivered.
YouTube video of Skype for iPhone making a call over 3G embedded above.
Update: 3G Skype calls may are only available on iPhone 3.0