HDTV

To connect a mac to an HDTV to output 1080i, 720p or 480i/p (using component video connector) should be easier to figure out, it took me some time, this is what i've learned:

Old computer, Old HDTV

1. if you have DVI, get the DVI -> VGA adapter, should have come with your mac, looks like this.
2. then you need an Audio Authority VGA to Component Video Converter. You might think you could use this (instructions), but the guys at ATI say it won't work, I haven't tried.
3. then you'll need DisplayConfigX.

Spend some time digesting the instructions for DisplayConfigX. It works, i've even tried it on the new intel mac mini. (I have yet to verify 1080i, but it doesn't seem like there is anything in the way of that)

I found another hint here.

New Computer, Old (analog) HDTV

if you have an older, analog HDTV that accepts component input, you're going to have to convert from your digital out to analog. the DVI / Mini-DisplayPort / HDMI to VGA adapters sold by apple WILL NOT WORK if you want greater than 480p out of your computer. The adapter you need is called "HDFury X3".

New Computer, New HDTV

Just use an HDMI cable