I thought Microsoft said that it will show up in Windows update around mid april. It will install automatically then and they claim that it's better to wait until then unless you have some urgent problem that you need fixed now.
Updating your Windows Vista–based PC
In mid-April, SP1 will automatically download and install on Windows Vista–based PCs that have automatic updating turned on. This is the easiest and most convenient way to update a Windows Vista–based PC, and we strongly encourage using this method. You can, however, download and install Windows Vista SP1 manually from the Microsoft Download Center.
MS has prevented SP1 from showing up in Windows Update if you have certain compatibility issues with SP1 that they are aware of. Check out this VistaBlog entry for more details;
Why SP1 May Not Show Up In Windows Update.
For me, it was because my audio driver and my fingerprint scanner driver were not compatible with SP1. After updating those drivers, lo-and-behold SP1 was sitting there as a critical update. For me, the download was about 122 MB and it took about an hour to install. I couldn't understand why it was taking so long to install, but when my system finally rebooted I quickly found out.

SP1 removed about 6 GB worth of "fluff" from Windows Vista. I now had an additional 6 GB on my system that I didn't have before. (It may even be more since I didn't get an accurate measure of my HDD Space before the update). I've also noticed my CPU fan kicking on less frequently, and some better overall performance. From the breif few hours I've had with SP1, I'm pretty damned happy with it.
As for the April thing, that is when MS plans on releasing SP1 to everybody regardless of their drivers. Basically, that is when they expect to have "Windows" versions of all the drivers out there that will be compatible. If you're like me and don't allow MS to update your drivers for you, then you may never see the SP1 update. So take a look at the link I gave an search for the drivers you have that may be preventing SP1 from showing up. Once you update those drivers to compatible versions, you'll see SP1 in Windows Update.
