Update: This article covers removal, see the IE9 Blocker Toolkit from Microsoft to keep it from installing. If you’re looking for registry options, the important values are DoNotAllowIE90 and DoNotOfferIE90 in HKLM\SOFTWARE\Microsoft\Internet Explorer\Setup\9.0.
I’ve had to remove Internet Explorer 9 from a variety of new PCs lately because they’re connecting to online services (hospitals, medical labs, radiology services, etc.) that are not yet compatible with the new version, including issues in Compatibility mode. I’ve also needed to provide instructions (now reproduced here) for some physicians to do the same on home PCs so they can connect into those same systems.