Giving away a great program like SketchUp doesn't seem to have won much goodwill for Google, and people don't seem inclined to cut them much slack when they screw up.

True, the latest update hasn't been rolled out very gracefully, but there is an actual update, the update installer does work, and once it is installed the startup notice does desist. On Windows, the new version number is 6.4.112, which Google is referring to as Build 4.

When the updater shows up on the SU opening screen, click the Download button; there is no other useful action. The information on the download page regarding the version number is incorrect. As I said, the new release is 6.4.112 (Win), as it does correctly state on the Release Notes page. If you simply go ahead and download the thing and install it, you'll end up with the new version and the startup nag screen goes away, whereupon you can start the program normally.

Presumably, Google will fix the faulty dialog and version number on the download page.