The bell is tolling for Windows XP, with Microsoft's plan to pull OEM copies as an option for system builders. In short, any PC bought from a retailer that has Windows must come with Vista - regardless of what the customer wants.
While you can already find a plethora of retail machines running Vista, there are still many people who for reason or another, be it lack of drivers for hardware or poor application support, would rather get a machine running XP. In January, that'll no longer be possible. Many businesses are still ordering machines running XP in bulk due to concerns over Vista, so there could be some backlash if the issues aren't addressed before then. Retail availability will probably still last for a while afterwards, but part of the reason people choose OEM is the cost reduction.