Windows releases new build of Vista SP1 to beta testers
Microsoft pushed out a public release candidate of SP1 last month. Unfortunately downloading and installing that version does not make you a member of Microsoft's inner circle, which means that you won't necessarily be able to download the new version through Windows Update or from Microsoft's password-protected download page.
On the other hand, some members of the Neowin forum have posted what may be a direct download link. This probably goes without saying, but download at your own risk.