The Microsoft Server team posted a quick article informing us that although SP2 will be available immediately as a high priority update on AU, it will not be forced onto machines for the first 3 months. If you wish to prevent this installation then you can deploy and manage a blocker tool for SP2. The toolkit contains an executable, script and a Group Policy ADM template. You can use either of the 3 methods to create and edit a registry key that blocks or enables SP2 installation.
The key is HKLMSoftwarePoliciesMicrosoftWindowsWindows Update. The value controlling the installation is DoNotAllowSP. Setting this to "1" will block installation of SP2 indefinitely via Automatic Update or Windows Update. The best solution to control this in an AD environment will of course be to use the ADM template and Group Policy.