I'm seeing a lot of posts on various forums and blogs with people complaining that SP1 although released has not shown up on their windows update.
Many are pointing to driver compatibility issues preventing perhaps being the cause.
According to an article I read on the Microsoft site (sorry not got source) SP1 although final and ready to install must be initiated by clicking a link on the SP1 page. You have to be viewing the page with Internet Explorer 7. Click here
The auto-update via windows update will start in April so the early adopters can beat the rush and get it now while the update server is relatively idle. Compared to the 2-3hr ordeal of the beta install the final SP1 took less than 30mins (once I had removed the beta edition).
I haven't tested extensively yet but I get the impression a some of the tweaks in the beta may have been left out of the final release to avoid compatibility issues so I am expecting a dip in performance compared to the beta edition I used. All I can confirm for now is that although I have 2 sets of drivers that supposedly have known issues - my sigmatel sound card and my finger print reader - SP1 works fine and I've had no sound or finger print reader issues since installing.
While it is always worth backing up and researching before installing any significant software or update I am fairly confident that the vast majority of users with mainstream hardware will have no problems. Self built systems with components form obscure manufacturers may fair not so well but they are like having issues with Vista in the first place.