O2 (in the UK) can’t be very happy?  I can now call any other skype user from my iPhone for free as long as I have a wireless connection somewhere.  It’s not as easy to receive calls, you need to be logged into Skype on the phone and you can’t run 3rd party apps in the background, but if you know someone’s calling you via Skype you can be ready for it.  And making outgoing calls for free to other Skype users is the real cost saving I guess.

Sound quality seemed pretty good as well.

I bet the Skype charges for calling real phones are cheaper than the O2 network charges as well, I may look into that.