This Ars Technica article about User Access Control in Vista is very interesting:

[Microsoft] does not view UAC as a "security boundary" ... UAC encourages developers (including black hats) to try and accomplish more without elevating permissions. Why? Because the goal should be to avoid tripping UAC except for operations that truly need elevated privileges.

This is why even users with Administrator permissions get prompted for UAC. An MSDN article explains why:

When an administrator user logs on to a Windows Vista computer, two access tokens are created: a filtered standard user access token and a full administrator access token. Instead of launching the desktop (the Explorer.exe process) with the administrator's full access token, the filtered standard user access token is used. All child processes inherit from this initial launch of the desktop, which helps limit the attack surface of Windows Vista. By default, all users, including administrators, log on to Windows Vista as standard users (my emphasis) ...

When an administrator user attempts to perform an administrative task, such as installing an application, UAC prompts the user to approve the action. When the administrator user approves the action, the task is launched with the administrator's full administrator access token.

However, if system admin privileges are not needed, applications can be installed in Vista without ever tripping UAC. The key to all this is using Windows Installer 4.0, which allows a number of installation scenarios which can be managed in a corporate environment using Group Policy and registry settings, etc.