Here in Austria there's A1 which afaik is the first mobile operator in the world to provide such a service (called "A1 over IP").

At a first glance, it is very interesting: same phone number tied to both cellular and ip networks, reception of normal mobile calls on the laptop, proxy servers at the same locations of those providing 3G access, good tarifs.

Practically, I still wasn't able to make it work on my E71's SIP client. I'll write back when I'll have a chance to test it.