I work as a Tech for one of the big notebook companies in Australia.
When our first & second wave Vista machines came out. There were literally no drivers available for downgrading to XP, Even directly from the chipset manufacturers! The most competent people would manage to get XP running but the sound/modem/lan/wlan chipsets were changed so the machines would be largely unusable.
Many Techs would call up almost in tears as they'd just procured 500 of these units and needed to roll them back to XP. I still don't see any certification or claims made on our new machines that guarantee 100% XP compatibility yet they still bitch and moan despite their own ignorance.
Its funny, for the 1st half of the year, We would get daily complaints and death threats from the geeks/techs wanting Vista drivers for their XP machines. Now for 2nd half of the year its been daily complaints and death threats from geeks/techs wanting XP driver for their Vista machines!!