Is this possible - cable in to vid card and then out to a TV ?

your question is a little vague. do you want to display the TV signal in a window from the pc? do you want to split the signal or use the pc as a passthru?

im assuming you want to be able to watch tv without the computer off.

im assuming you want to view this at 720p (HD) or above.

im guessing they want to be able to record the output from the cable box.

in which case you'll need a fairly beefy capture board to do this.

theres a couple of inexpensive options
http://www.hauppauge.com/site/products/data_hdpvr.html component video + optical audio. and it records directly to h264. - I'll be investing in this one soon.
http://www.blackmagic-design.com/products/intensity/ HDMI - im not sure if comcast currently enforces hdcp across the HDMI port or if they plan on doing so. if theres no copy protection across thier HDMI signal then this is the best bet. if there is then this may not work at all. if i didnt hate RMAing stuff i'd get one to play with. this setting may also differ between models of the set top box and between broadcast areas.

if you want uncompressed 1080i or higher input for "Digital" cable channels ( anything higher than channel 99) then it gets expensive fast.

some models of the comcast boxes support firewire out. i couldnt get mine to function as they killed the sync pin making the 1394a port on the back useless for anything other than a hard drive.

if you find a different inexpensive capture board that will do 1080i w/ mpeg2 or h264 hardware compression let me know.

Any idea if I would have to worry about copy protection with that unit?

not unless comcast disables the analog out on the set to box. ( not in the forseeable future)

Will it display only the picture coming from the cable box on the TV or will the TV simply display the same image that is on the computer monitor?
that depends on how you cable it.

theres video out ports on the hauppauge device that display the same signal that came in.

but you can take the video out ( VGA/DVI/...) of your video card and display that on the tv and have the signal from the hauppauge unit displayed from the pc.