Barry Goffe from the Ultimate Extras team just posted that Windows DreamScene has officially RTMed. No word yet when it will be available to download, but as soon as I know I'll pass it along.
Be sure to also check http://dream.wincustomize.com for DeskScapes, the add-on for DreamScene that lets you get crazy with dynamically-rendered content.
It's not the big Ultimate Extras strategy and featureset update that everyone was hoping for, but Barry's post alluded that more news is coming.
UPDATE: Apparently it is there for Windows Vista RTM customers, but currently anyone who just upgraded to the SP1 Beta are SOL.