Skype have updated their iPhone app to v2.0.0 [iTunes link], which now supports Skype-to-Skype VoIP calls over a 3G connection. According to Skype, the functionality - which routes voice calls over the data network, rather than using your regular carrier minutes - will be "free until at least end of August 2010"; after that point, they will be charging "a small monthly fee" for the service.
Exact details of the monthly fee are currently unknown, nor whether it will cover any additional features beyond 3G calls. The news follows on from Qik's quick denial that their two-way video calling app for Android would require a subscription for HTC EVO 4G users; they're insisting that such calls will always be free, though they too plan a premium subscription for so-far unannounced features.
Also new to this release are an update to audio quality in-app, which Skype say is "near CD" in fidelity when used on the iPhone 3GS and 2nd-gen iPod touch, and a number of UI tweaks. The screenshot above, incidentally, is taken from the iPhone Skype app running on an iPad.