I’ve been using Rebtel for a couple of months now to call mobile phones in Spain from my mobile phone here in France. It’s rather complicated to understand how it works I think, but here goes:
1) subcribe to their site, pay up front some money
2) use their service to create a LOCAL number for an INTERNATIONAL number. you can do it on a permanent basis, or a temp basis. I’ve added a few people I used to call on international numbers with rebtel numbers, and added those to my contacts list on my phone 
3) you’re charged very low rates (local calls) + $1 USD/week. If you call often then you make money. The $1 USD seems like a lot it you only place ONE call a week.
4) there’s a callback mechanism where when you call someone, they pick the phone, hang up, and call back. And then the international call is free.
All of this, while we wait for Skype to come up with a mobile version … (I’ve seen it with my own eyes already , although it’s not for public release AFAIK).
