I discovered that, those pesky screen blanking intervals that occur when ever UAC is triggered are caused by an interaction between "Desktop Composition" and the "Secure Desktop" element of UAC.

Either unenabling "Desktop Composition" or Disabling "Secure Desktop" will make the multi second screen blanks surround UAC dialogs go away. Vista will simply darken the background, and bring up the UAC prompt. Just what it should be doing.

Disabling Secure Desktop is not good, Secure Desktop prevents background tasks from interacting with the UAC dialog.

Desktop Composition is responsible for the translucent glass look of Vista Aero. Setting your theme to Aero Basic or Windows Classic also disables Desktop Composition.

I stumbled on all this while doing a clean reinstall of Vista Ultimate and the machine initially came up with a vanilla VGA driver that didn't support the translucent effect, AND didn't have those annoying blanking intervals.

edit: P.S. Turning off Desktop Composition boosted my Windows Experience score from 2.4 to 3.0.