The slow rollout of Vista SP1 has been going on in limited fashion for a short while, with the complete rollout not finishing until April. However, many users running the 64-bit flavor of Vista have reported that they are able to update
normally via Windows Update. The update size could be huge, upwards of 700MB, and isn't yet available for all Vista users. If you have the chance, however, now might be the time to grab it.
That is, if you're willing to take the risk. We've heard various bits of bad news
regarding Vista SP1 lately, including Microsoft having to halt distribution
of some prerequisite updates due to bugs encountered. Service packs are always an adventure, it seems.