I don't have an iPhone, and I have limited experience using Skype on my Android, only for IMs... but you might want to make sure that Skype is only using WiFi and not any ridiculous expensive roaming data.
Also at some point I was under the impression that Skype and/or similar services use phone minutes if you are calling another phone, and data if you are calling a Skype account. I think it's enforced by the terms of service for the APIs needed to develop software on the smartphone platforms.
If I were you I would do my research and make sure I understand what my phone is up to. Wouldn't want to rack up ridiculous charges while thinking you were connected for free!
After looking at this
it seems like you just have to make sure you are on WiFi and that it's not using 3G for data.