CableCard provides the digital mapping information. CableCards tell the Tivo what channel is what so they can provide it with guide information. You need to use CableCards to get guide data on digital cable channels with the TivoHD.
Many cable companies receive their local channels via fiber which [often] does not include PSIP information, so that is not a reliable means of determining channel numbers.
As for what sort of subscription is required on TWC, I can't answer that.
You do always have the option of an antenna. The Tivo will integrate HD locals from an antenna (with guide information) with your cable channels into a single guide.
Many cable companies receive their local channels via fiber which [often] does not include PSIP information, so that is not a reliable means of determining channel numbers.
As for what sort of subscription is required on TWC, I can't answer that.
You do always have the option of an antenna. The Tivo will integrate HD locals from an antenna (with guide information) with your cable channels into a single guide.