I've been waiting for this one for awhile, and thought it might come from someone like Skype...Skype has introduced the Open Sky - a service allowing you to directly link your VoIP system into Skype via a SIP trunk.
Soooo...if you're a true geek and run a VoIP home system (I use CCM or CCME), you can now directly link to Open Sky via SIP and not require a typical telephone provider to make outside calls.
The catch? You can only call Skype users...ehh...and only for a "short duration" depending on how busy things are.
So...almost there. Someday, we will have free PSTN calling through one of these types of service.
Here's the link with the information on OpenSky.