A lot of larger corporations have very long upgrade cycles. Several that I know have 6-8 year upgrade paths for the OS. The fact that XP was around for so long is the only reason it was adopted in such numbers. Most corporate IT types didn't want to switch from Windows 2000. They couldn't see the use of the colored task bar and curved dialog boxes....sounds a lot like Vista today.
If MS is successful in returning to a shorter OS development cycle it's certainly possible that Vista will show lower penetration in the corporate market. The biggest thing holding it up now is the higher hardware requirements.