It seems that Microsoft may be a victim of its own success. Windows XP is quite a reliable desktop operating system, so why should organizations spend vast fortunes migrating to Vista? Further, there are many application compatibility issues and questions (on one of my home systems where I run Vista Ultimate, several programs do not run at all or run improperly).
So it may be of little surprise to learn that U.S. government agencies, including NIST and DOT/FAA, are banning Vista, for now.
NIST bans Vista (Dark Reading)
U.S. FAA bans Vista, IE7, and Office 2007 (Information Week)
the reason why, perhaps.