Microsoft put a document for download here. They briefly describe that this is going to be a smaller SP (compared to XP SP2)  but it will address many issues that customers reported.

 

I'm looking forward to this update, since i actually like Vista, but also encountered lots of "issues". I won't install a beta SP on my Notebook that i use for work, of course. But in the near future i would love to do projects with the new Vista desktop features like the new search engine. This will not happen until the corporate IT departments of our customers have full trust in Vista. MS must have noticed the reluctance, and this is definitely the best way to react on it.