SP1
I know several people who have a valid licence of Vista, but have not installed it yet - or have installed it, but have given up and gone back to XP. The reason? Vista is still essentially in Beta testing stage.
When SP1 comes out with its bug fixes and its driver updates more people will wan't Vista. If MS act smart they will also update Vista according to the complaints people have made about the way it works. Vista 2.0 instead of Vista 1.1.
What will also be holding back Vista is that most companies will not want to use Vista for probably another 6 months to 1 year. They will buy HP or Dell instead for now as they still come with XP if necessary (this is exactly what we have done). Acer should dig their heels in with MS and do what the others did - stop making Vista a definite and make it an option.