Service packs are a lot more than just aggregations of all the current updates (wow, I used "aggregation" in two posts in a row, I need a thesaurus).
Service packs usually include altogether new builds of a lot of the system and supporting components. Windows XP SP2 was a particularly exaggerated example of this, because they really had to rewrite lots of components to secure the system. Some service packs are smaller and simpler, but they still follow the same idea of being an updated version of the OS.
I for one, don't think they are holding updates off because of the service pack. I've gotten a couple new drivers recently. For the most part -- there actually have been substantially less security problems with vista than xp in the same period (excluding the 64bit signed driver debacle).
They have been releasing a lot of hotfixes to limited audiences. I think the reason for this is the newness of a lot of the system. They are having to really test out the causes of some of the issues and what effects their fix has on other things. For the most part, these aren't usually things pushed in updates anyway since hotfixes are only slightly tested, and pushing them to their population could really cause problems. And that is what issuing a service pack is for - finally delivering a major system update of tested code to the production population.
Cheers,
Rob Redbeard
