Microsoft has a huge problem with users not upgrading until they buy new hardware. The number of people running Windows 98, Windows 2000 (on the desktop) and Windows ME is staggering, given how old they are and given how much more stable XP is. My yawn at Vista is based on the assumption that this trend will continue, and that fewer than 50% of all Windows users will be Vista users until 2009 or 2010.
If then. Many users upgraded from earlier Windows OSs to XP because XP got a reputation for being much more stable (that is, it didn't crash during a normal day's worth of use). That reputation has since been tarnished by XP being much more susceptible to security attacks. Vista, on the other hand, may not be much more stable than XP. In choosing the "Vista" name, Microsoft is pushing the idea that this is a new way to view your work. Note the "view", not "perform".
Very few Mac users went from MacOS 9 to OS X because it looked so much nicer. (Well, some folks will buy anything "lickable", but they are a small minority). They bought it because it got a reputation for being more stable and having many more intuitive features. From everything that we've seen so far about Vista, it is not more intuitive; in fact, some of the UI features seem like flash-over-function with no good reason other than "OS X looks a lot better than XP".
Unless Microsoft gets a lot of "Vista is better than XP for things other than looks" reviews in the first few months, the 50% adoption point could easily be more than six years from now.