If you were one of the ones who ran into trouble installing the pre-requiste updates that are required to install the Vista SP1 update, it looks like Microsoft may have solved the issue. From the Microsoft Update Product Team Blog comes this:

So what caused the problem?  Well, the SSU has special code to check whether there are any pending reboots or other updates to install.  If it sees either of these circumstances, it prevents the install from starting.  During our investigation, we discovered that there were a few unknown and rare events during the middle of the installation of the update that could cause the update to think it needed a reboot to complete the installation. If this happened, the system entered a repeating reboot loop.

To address this problem for people who have not already installed the SSU, we are releasing a fix tomorrow which will install prior to the SP1 Servicing Stack Update.  This pre-SSU update helps to ensure a smooth install of the SSU by working to prevent the system from rebooting during the SP1 SSU installation.  We also made additional changes to the SSU installer code, so that it checks for and requires the pre-SSU (KB949939) before it will install. 

Check out the full post here.