I've not had a problem before or since SP1 - Vista close down is very quick. What services/ processes have you got running in the background. Is it having to finish some silent task you're unaware of ?
One of the first things I did after installing Vista was to turn off all its Search Indexing. It really annoyed me how much time the system spent doing work for this function. I know where I've put everything, I don't need Vista chewing up the bulk of my resources to keep unnecessary encyclopaedias about it.