If you are referring to DSL VoIP yes, they can tap it. DSL runs over a copper loop (i.e. a phone line). It's just provisioned differently at the central office to use the traditional wasted band(width) of the copper pair.
As for cable, I would say the answer's no. Not at this time. Copper is shared, true. It's on a loop shared by your neighborhood, or area. This both creates the problem where someone on that loop could hack in (in thery) but would also make it harder to tap if there were say, 100 cable users on a single loop.
Nextel's can't be tapped either because of their encryption.
However, with systems like Escelon, and so forth they were designed purposely to track all forms of media.. phone, internet, e-mail, and so forth. Some have complained that the system is being misused and being used for corporate espinoge in the E.U. countries for industry secrets.
I guess the best answer for your question would be to do cable (like a Vonage for consumer), and get some additional encryption software/hardware.
