There are already a few apps (such as Nimbuzz and Fring) on iPhone that provide Skype VOIP phone calls when you are on wi-fi. But they aren't the best because they don't show contact status properly and sometimes take a minute or so to log you in. People have wondered why Skype hasn't released a version for iPhone when they provide versions for so many mobile devices.
According to this:
"Silverman refuses to be drawn on his specific plans for Barcelona. But industry watchers reckon Skype plans to release a version of its software for Apple's iPhone as it gears up to do to mobile what it has done to landlines.
Under Silverman, Skype has been building steadily on its products for mobile users. It now works with phones based on Microsoft's Windows Mobile operating system, and a slightly more basic version is available for owners of LG, Motorola, Nokia, Samsung and Sony Ericsson handsets. The company also has a partnership with the mobile network 3, with a range of jointly branded phones. "