By performing the update via a machine-based group policy, you are assured of a quick and uniform distribution. MSI packages have the ability to automatically elevate user permissions sufficiently to permit installing applications. Update.msi is an example of this type of package. By elevating permissions programmatically, there is no need to have desktop admins perform the installation. If you want to distribute SP1 manually, you can e-mail out a shortcut to Update.msi or install a desktop icon as part of a logon script.
Standard users can run the package thanks to the permissions elevation.
If SP1 has already been distributed to a desktop via a group policy, it will not be installed again. Deleted or damaged components that are not already protected by the Windows File Protection system will be repaired by the Windows Installer service. MSI packages can be de-installed quickly and easily. Users must have administrator rights on their local desktops to remove the package. Distributing installation packages via group policies is quick to set up and simple to manage.