Skype is now available for the iPhone - and will be coming to BlackBerry in May.
The VoIP app on the iPhone will only work over Wi-Fi and not 3G.
It will allow iPhone subscribers to use phone numbers in their existing iPhone address book - without the need for duplicate lists.
UPDATE: Since being released on Tuesday it has emerged that Skype on iPhone does works over 3G, despite being intended only for use with WiFi.