Recent updates or newly installed programs may do that but not each time just the first time. The general issue with slow shutdown is a hanging program wich will not close such as email checking your in box..
There is definitely something taking way to long to shut down with Vista. I am seeing the same issues here and have seen lots of similar posts. One of the most logical things would be with the way Vista handles administrative control of hardware and services during shutdown. Who knows? It is definitely not a software update, third party app or something everyone will try to pass it off as the first time around. Hope to that Microsoft will acknowledge an issue here and send out a fix. Other than the shutdown thing I have been liking Vista. I also run VMWare Server and Ubuntu on the same laptop since some tools for work are not available on Windows.