Mac OS X - watch cable over firewire?  

There are two ways to do this.

1. the elgato eyetv 500 can decode HD cable in the clear (real-time), or
2. Record using VirtualDVHS and playback using VLC (up to 10 second delay).

A VLC module is being developed which reportedly works great for SD but needs some help for HD. See here

Actually I have been using number two quite a bit but not for cable, but for AVX1. It works very well but its a bit out of sync because it takes you a little while to get VLC going, in my case usually more than the quoted 10 seconds. The quoted module doesn't work yet.

But you might consider a third alternative -- my Dell computer monitor has PIP, use the Svideo drive output from the cable box and just drive the PIP.