Skype is now live for the iPhone in the US! But probably best not to grab the app from the iTunes store yet though.
Reports have come in that it causes problems for units not on OS 3.0 Beta, particularly those who try to connect via 3G. These units experience audio loss as well as frequent crashes. Those who try to connect over WiFi are fine though, and Engadget reports that the "audio quality was surprisingly good, far better than any third party apps we've used."