Now this is a pretty good idea. You can dial a local access number (PSTN) and then enter in a SIP URL/address which will then connect the call for free. SIPPhone is offering this service.
You can view the complete list of access numbers here:
SIPphone - Call worldwide over your broadband Internet connection with any phone
Finally, I should mention that SIPPhone has some interesting case studies that involve universities using their products. VoIPNuke's Skibare shared this info with me. I'll blog about them in a few days or so.