Previous poster(s) hit the nail on the head... very little incremental cost, if any, to put XP images on their machines, vs. Vista unless MS' bulk price for XP licenses has gone up.
I'm surprised that a handful of people have defended Vista as performing reasonably well, and stably. I fix and set up PCs for home users, and I have yet to see a Vista machine, whether bargain basement warehouse club cheapy, or high-end gaming rig, that didn't pause at odd, arbitrary moments during simple operations such as opening up a folder, or populating the control apps in the Control Panel.
The performance issues I described are after I do a thorough performance tuning - putting it in Classic Mode, removing bloatware, using MSconfig to disable all startup items other than the security package, and disabling unncesessary services.
I've done perhaps a hundred of my own vista ----> XP "downgrades" (Had customer buy an XP CD) and they've gone rather smoothly, resulting in far better performance. The thing I feared since the day after the official launch (the day I did my first downgrade), are manufacturers that are making OEM devices that go into system boards, such as sound, networking, etc without publishing XP drivers. So far, not so much, with the glaring exception of a Dell Studio laptop with a Broadcom wireless device for which I could only get Vista drivers.