A wealth of information in NY Times article on how to use Skype with your mobile:

Both SkypeOut and SkypeIn carry a relatively low fee. SkypeOut calls to land lines can be as little as 2 cents a minute, while calls to mobile phones are usually a bit more. In Italy, for example, Gary’s call cost me (not him) 30.8 cents a minute (not including tax). A SkypeIn subscription, meanwhile, costs $60 a year or $18 for three months. All I have to do before I leave home is set my American cellphone (an older-generation iPhone) to forward to my SkypeIn number, and all I have to do when I arrive in a new country is get a SIM card, go online and set the Skype software’s preferences to forward all calls to the new number.

So, here’s how Gary’s call to me worked:

He dialed my regular American cellphone number, which forwarded to my SkypeIn number, which, in turn, forwarded (via SkypeOut) to my Italian cellphone number.