Way back in 1999, I was installing voice over IP systems for enterprise customers paying several hundred thousand dollars to replace their existing hardware. The software worked, but remained victim of the frequent reboots required by Windows NT 4. Services have improved since then, but the technology is still overpriced in many markets.
Four years later, the P2P guys figured out they can tie existing VoIP technology together with P2P to make a killer phone system - Skype. And they are doing it for free. Finally, a use for P2P that won't have the RIAA beating down your door (unless Skype starts including on-hold music).
Skype leverages the talents of two guys from the Kazaa team, offering a service that makes it simple to connect with any of your friends who also have Skype installed on their system. At the moment there's no integration with the traditional land and cell phone services, but there is virtually no need.
Sure, you still need a cell phone for travel, because this will probably never work in the car, but it could easily replace your home phone as a means for communication on broadband networks. The ability to see your friends, business associates, and relatives online means no more leaving "hey, call me back" messages. Just wait until you see someone you want to talk to online, then call.
When VoIP via P2P technology hits a point where you can call friends using a headset, PDA, and Skype software, using a wireless network, existing phone services will feel the financial hurt. I doubt P2P will kill either cellular or landline phone service, but it will certainly shift some dollars from the pockets of businesses that have far too much control over the way we communicate - I can't wait.