I will most definitely wait a year or more before buying Vista. Because things will probably fly the way they did when XP first came out. That is, full of bugs, getting hit with new viruses every day, huge services packs coming out, incompatibilities with programs, not so great support for old hardware still in use, system requirements not being met by upgraders, etc, etc, etc.
We didn't start selling XP with new systems until after SP1 came out, had to many issues with motherboards and CPUs not being supported right.
I would expect the same outcome, if not more so, from Vista. Being that for the most part it will either be missing features, or have beta software installed. Think about it, brand new graphic core, brand new file system, brand new IE etc. A years worth of bug fixes and a nice, fat, service pack. Then maybe I'll jump on.
And of course, it would be best to have Vista on a x64 platform.
3 years ago the mantra was, "get XP on a new system, but don't bother putting it on your old box unless your hardware is less then a year old."
I expect the same with Vista. Get it on a new build, or if you have newer hardware.
That's just my opinion.