The latest report with the highest numbers I've heard was:
6.8 million downloads, about half of whom, or 3 million or so, are electing
the Spynet reporting option.
From here on out things get muddier because of the requirement to
re-download to get the .509 build. An earlier Security360 webcast stated
that this download set a record (I think for one day downloads) for the
Microsoft download center.
A fair proportion of those users haven't found the need to post here, and
its a good thing!