Businesses often use Software Assurance, a licensing plan from Microsoft, to manage their Microsoft licensing. Unfortunately, Windows Vista Ultimate isn’t available through Software Assurance. Ultimate really isn’t intended for businesses, because it has many features that you can’t manage by using Group Policy (like Media Center). But, Microsoft knows that IT guys don’t want to be told they have to use a limited feature set, so they’re giving Software Assurance customers a way to get Vista Ultimate.
Basically, have your Microsoft Volume Licensing administrator head over to your volume licensing site and order the Vista Ultimate media (you can’t download it).
Detailed instructions are available from Eric Ligman here and here. Eric also provides instructions for downloading Vista Enterprise (if you’re a Software Assurance customer)–the version of Vista that really is intended for business use.