CableCARD 2.0 is reportedly coming in 2008 and will bring us two-way interactivity along with support for six or fewer TV tuners (unique channels can be tuned). The current CableCard 1.0 that's been talked about so much and is included in the current batch of CableCARD, VistaPC's won't be upgradeable to the newer 2.0 version. Same story for the Tivo Series 3 hardware. That means you would have to again buy all new hardware to get the 2.0 version.
Why would you not want the CableCard 2.0 features like multiple tuners (up to 6) and PayPerView among other things? Well mostly because this version of CableCARD would force the Cable Companies UI on you instead of the nice Tivo, VISTA or whatever PVR app gui you are used to seeing. Seems like its full of potential problems just like the current CableCard version. None of this is certain at this point since there is still bickering and arguing between CableLabs and the Computer Electronics companies like Tivo.
You can read more about CableCard in this article at Electronic House