Serious companies have downgrade rights that allow them to install their own image on OEM-supplied computers. They may come with Vista, but companies that need to will still be able to replace them with XP.
In reality, I think you'll find that larger companies with mature processes will move to Vista because there is a good business case to do so. The rest will migrate on their own time, or not at all. After all, there are still companies out there on NT 4.0.
We still have a bunch of Windows Professional 2000 boxes, and by far it's the most stable Windows version ever.