Hi Mike,
actually it is feasible in NT4 - I know this because Robbie did it for the
www.biodiversity.co.za site.
It's done using css and divs and so on.
that said, it's not pretty. The short answer is that horizontal centering is a grey-area with web browsers. And the results can be very different from one browser to the next. There are lots of clever hacks out there which can give the illusion of centering, but they're usually quite specific to a page layout. In other words the css will vary from site to site, and indeed from page to page. So there's no really "generic" solution.
also CSS tends to work at the "page" level - so if you go down a particular CSS road, you tend to then need other CSS on the rest of the page.
On the up side the browsers are getting better - ff, chrome, safari, opera and now IE 8 are behaving more or less consistently. IE7 requires a different style, but IE6 is basically hopeless. So then your site falls into the category of "if you want it to look good, use a modern browser".
In summary - you can do it already, and I'll work on some NT5 examples to show how. But it won't necessarily be universally centered for all browsers.
cheers
Bruce