Yup, you can.

Microsoft Vista Business & Vista Ultimate come with ‘Downgrade’ rights.

IBM have a page explaining the process, as do a number of other suppliers.

Why would you want to? Well one client I have is about to purchase some new workstations, rather than upgrade all the other workstations to Vista, or have some running Vista and some running XP, it makes sense for all workstations to be on the same OS, with the same apps. By downgrading to XP now, and having the right to revert back to Vista later makes for an easier transition now.