Actually PTC is a US Government regulatory specification. It can be realized using multiple technologies. FRA requires that each realization be separately evaluated and certified as compliant by it.
Amtrak realizes it using PRR coded track circuit cab signal + ACSES. This technology can be purchased now from multiple vendors.
Most freight railroads and most non-northeastern commuter railroads are realizing it using what is broadly termed as ETMS which uses GPS in conjunction with other position information that may be available and integrates with the extant signaling system that is already in place. Again this is now available from multiple vendors.
California HSR plans to realize it using an ERTMS2 implementation, probably from some European outfit.
You are absolutely correct in stating that the core technical pieces have been around for a while. But typically the devil in these is in the details and that is where each implementation potentially trips up. NYCTA has mightily struggled with deploying CBTC, which they have not quite got to work flawlessly yet, and yet CBTC has been deployed elsewhere quite successfully. The motivation for using a GPS and radio based system was that the cost of installation and operation in a far flung network was believed to be significantly less. that is yet to be seen.
Interestingly railroads like UP already have a form of cab signal deployed on significant parts of their Overland Route. So the mere hookup with their current signal system will already give them position information about trains. I am curious to see how they integrate the GPS based system with their track circuit based signals in the back office system.
To me at least neroden's claim that everything should already be running like Skytrain was a bit off the wall since no main line in Europe or Japan even quite do that, though they can come close. PTC as currently specified by the FRA in the US is certainly not targeted towards such either.
