It's no use trying to pretend otherwise, we didn't do a great job on making Vista SP1 available. Some of us in the field (Keith for one, Matt for another, Daniel for a third) felt the heat from customers and we did our bit to get the update out sooner. There was a lot of frustration internally.
Well if you're a technet subscriber the Update is at http://technet.microsoft.com/en-us/subscriptions/default.aspx. (Ditto for MSDN) You might expect it to be under downloads and product keys. It isn't (at the moment). Kathy has posted more information to the Technet Plus blog.
The slipstreamed install version will be posted soon. After what Mike Nash said I would have thought that would have be first - but what do I know ?
Incidentally there's a "notable features in Vista SP1" document which gives you a run down of the changes. Most of it is made up of a listing of the hotfixes which came out before SP1 and have been rolled into it, but also gives an idea of what's SP1 specific.