They'll be forced to upgrade if they want to buy new PCs, and then you'll have to maintain both XP and Vista boxes, that's when they decide to upgrade to Vista. That's how MS gets you, if you want new machines they come with Vista installed, you don't have a choice.
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.