This isn't the final version of Vista's upgrade, but the general public can now get their hands on SP1 RC Refresh right now.
Go here to get it.
This consists mainly of bug fixes and performance enhancements, with no major new stuff.
Wired warns, however, "
bear in mind that SP1 RC Refresh, as the update is known, requires previous versions of SP1 to be uninstalled first, and there are several updates you’ll need to apply to your existing Vista install which can’t be removed later should you want to downgrade to a stable version of Vista."
The final version of SP1 should be ready sometime next month.