There are reports of people not being able to get Vista SP1 via Windows Update. While this may sound weird, that’s a good thing because one of the things that Windows Update will do is check to see if you have anything on your system that may cause a problem when upgrading to SP1. Microsoft has a document on the reasons that cause you not to see Vista SP1 that you should read if you run into this problem. The most common reason seems to be driver related (as a lot of Vista issues are caused by drivers that are not up to snuff) and Microsoft has a list of drivers known to be bad actors in the document that I referenced above. Most seem to be sound card related, but there are also reports of Nvidia drivers doing the same thing (although we have several computers that run Vista with Nvidia cards and we have not seen this in house).
The question is, why is Microsoft doing this? In short, it doesn’t want to make Vista any more hated than it already is. They’ve actually listened to feedback from customers (read: corporate customers who pay them a lot of money in licensing fees) and have built SP1 to inflict as little pain as possible. It checks your system up front and refuses to install if it thinks that bad things will happen. So the bottom line is this: If you’re inconvenienced for a bit while you update your drivers and get your system ready for SP1 to install properly, then in my humble opinion that is better than SP1 trashing your system. Sure there are ways around this, but if I were you I would not use them. After all you want your system to work properly, right?