My (work) laptop dual-boots between XP and Vista, and although Vista has some nice desktop widgets and things, I tend to stick to XP whenever I can. Vista is a resource hog, especially in terms of memory, so a machine that runs XP perfectly happily, with all my dev tools running, is painful on Vista. Without going into too many technical details, Vista appears to use a different memory management policy to XP, which is inferior for my usage. It also has a couple of irritating 'features' which I just can't get on with. No doubt many issues will be sorted in the service packs.
As regards 'downgrading' to XP, I would be surprised if anything in a standard laptop available today doesn't have an XP driver available. The very high-end, bleeding-edge ones might do, but even then, it's unlikely, unless they have Vista-specific features (like sideshow). I would suggest that you take some time to assemble all the necessary drivers from the 'net before you start, though! Also, be aware that there may well be warrenty issues - there was some recent press about a chap who had a hardware failure, only PC World wouldn't honour the warrenty because he'd installed Linux...
Unless the supplier is prepared to swap it out for you, you'll probably end up buying a copy of XP anyway. They are available retail, as others have said.