The FCC has released their regular stats on telephone penetration in the USA.
They also issued a report on how much people are paying. So right now VoIP appears to be a "cheaper" way to go, but I don't expect that to last. While I do forsee the day of $9.95 plans, I believe that pricing of VoIP will eventually mirror the current pricing of PSTN, and will have a downward pricing pressure on PSTN. That said, as more advanced services are rolled out with VoIP, that history will repeat itself and the add-ons will cost more money, the same way Voice Mail, Three Way Calling and Call Waiting are charged for.
Now if they would only break out VoIP numbers in their report it would be interesting to see the demographics and segmentation figures.