I you wanted you could get a TV tuner card. Then you stick you video cable into it.
Yes, there is the Miglia PCI tuner card, or the El Gato Firewire solution. BUT...they don't do HDTV, just old-fashioned NTSC. And HDTV is God's Gift to football.
I hope someone can convince ATI to make a Mac version of their upcoming x800 based All-In-Wonder. Besides, the x800 is more powerful, but burns less power than the nVidia card. This is important if you don't want a noisy computer. Right now my dual G5 is pretty darn quiet, thanks to 9 (!) slow rotating fans.