Interesting article from ComputerWorld about UAC improvements and a reduction in prompts to the end-user by the operating system.
“Among the changes Microsoft's made to UAC that will drive that decrease, Cooke cited several specific examples, then pointed to the new "slider bar" that will let users fine tune the intensity of the security feature. "We've reduced 16 different points of prompting," Cooke claimed, ranging from allowing a standard user to pull operating system updates without seeing a prompt to viewing (but not changing) firewall settings without a pop-up. Nor will Windows' own components throw up a UAC prompt, Cooke added”
UAC has been deemed a success in Vista because it has caused Microsoft’s vast ISV community to think about “Standard User” as the default security threshold to run applications. Most of the common security threats from the XP-era simply go away in Vista because of the fact that users are set up with standard user rights where as most users had administrative privileges in XP.
Windows 7 expands on this but allows for a customized user experience so that organizations and individuals can set their own warning level.