You can capture over the firewire port, but it's been a while since I've done it. and i've never used a scheduling software to do it. just to manually record some clips of a friend on TV (had recorded the show on my Comcast DVR), that i then dumped to DVD.
other than that, currently I don't know of anyway to capture a HD signal from an external box over component, DVI or HDMI.
Although it isn't HD resolution, you should be able to do what you want with s-video and currently available capture cards. it will still be a decent picture and in the 16x9 format, although letter boxed. i'm not too familiar with the mac hardware/software offerings.
I use mythtv (linux based) for watching stuff in the bedroom, plus it automatically dumps shows to my iphone for watching on the subway or wherever. and i can stream live TV over the wireless network to my laptop.
I just use a DVR from the cable company to record the HD stuff