cnet News just announced that Skype for iPhone will be released on Tuesday, March 31. This means that iPhone users will be able to make VoIP calls with their iPhone over wifi networks. The app will not work on cellular phone networks. However, I assume that a jailbroken phone with the installed VoIPover3G would work to get around this technical limitation.
In any case, I’m looking forward to trying Skype from my iPhone.
Also relevant and recent, read why “Skype May Be The Biggest Winner From The Web 2.0 Era” (ReadWriteWeb).
Update: So it appears that Skype will be available just about everywhere, except Canada.