I purchased the least expensive calling plan on AT&T to use with my iPhone since I still have a T-Mobile contract with lots of minutes for most calls. It is a bit of a pain to manage those minutes though when I carry the iPhone around with me all the time so I was pleased to read the Macworld article that announced Skype can be used on the iPhone with IM+ for Skype from SHAPE services. IM+ for Skype is currently in public beta.
Simply point you iPhone Safari browser to www.skypeforiphone.com to get started with the client application. IM+ for Skype works using SkypeOut services so you will have to make sure you have SkypeOut credit on your account. The website states that IM+ for Skype will work even if you don't have a WiFi connection, but I will be curious to see how well that works with the slow EDGE connection. IM+ for Skype costs US$14.95 for other platforms so I imagine after it comes out of beta on the iPhone we will see this same single application fee.