A leaked version of what is being reported as SP1 for Vista has been floating around for a while now. APCMag takes a look at this first look of the new features. Here's an excerpt:

First up, I can say that there is a very noticeable performance increase. It is obvious that since Microsoft released Vista to manufacturing, it has been optimising the code ever since. (I suspect this revelation will fuel the fires of the people who say Vista was released before it was ready for prime-time.) There's far less hard drive thrashing and in general the system seems much smoother and more responsive.

Normally, service packs don't include new features - Windows XP SP2 being the standout exception. By and large this seems to be true of Vista SP1, although there is a new maintenance application called "Create a Recovery Disc". This isn't the same thing as creating a backup of the system to DVD - instead this process creates a recovery DVD which you can use with system recovery options if you don't have an original Windows DVD or you can't access OEM recovery tools.