No, if you're a cable subscriber, you need a tuner for your computer that accepts a CableCARD. Tvheadend doesn't really support CableCARDs, but there are workarounds. I am successfully using 3 SiluconDust HDHomerun Prime receivers (each supports 3 tuners, so I can receive and/or record 9 simultaneous streams/channels).
However, it is a little bit of work to get it going properly. MythTV offers better support for CableCARD tuners; but it can work with Tvheadend.
In short, though, No you cannot just plug in your cable feed. (Especially with Comcast, who encrypt everything and mark more channels than necessary as Copy-Once, meaning your only way of watching those channels is with Windows Media Center.)
