My memory is shaky on this, but I believe quite a large number of machines with OEM Vista licenses shipped with XP installed, via downgrade rights. I know the organizations I was working with at the time bought quite a few HP machines configured this way. MS kept extending XP's availability in this way, somewhat masking the almost complete lack of interest in Vista for corporate clients.
What failed to happen was a mass migration to Vista by corporations. They mostly just waited for Win 7.