I had been thinking of that, though Twilio's pricing would probably be prohibitive. They actually charge more than my cell provider for minutes. But everything you'd need is right in the API.
My concern isn't really about the money, though the free calls through voice are nice. It's that Google voice has my number now (as a gizmo refugee), and that + a voip provider + cell provider could be condensed down to twilio on the desktop and wifi, and maybe cell data on a tablet, or prepaid cell as a backup.
The $1/month for a number isn't bad, international sms is awesome for my use case.