Big change yesterday. Until now, if you wanted to use Skype or another voice-over-Internet app that lets you make free or low-cost calls via the Internet, you had to do it via Wi-Fi. You couldn't use your iPhone's data connection. Yesterday AT&T announced that their policy had changed. People had long wanted this change because, for example, they can make international calls a lot cheaper using a VoIP service. This article from MacWorld gives more information.