1. Use an environment with mixed DCs of both 2003 and 2008 or 2008 R2 Servers
2. Set your GPO, using the 2003 Server Group Policy Management Console (GPMC) as follows:

Computer Configuration\Policies\Administrative Templates\Windows Components\Windows Update\

No auto-restart with logged on users for scheduled automatic updates installations = Enabled

3. Apply the GPO to a test server, deploy new updates via WSUS and wait for the server to download them for scheduled installation later.
4. Log on to that server via Remote Desktop, verify that updates are waiting to be installed. Log off.

Now here’s where it gets interesting: The server will immediately restart now.

There’s probably a couple more things that need to be set for this issue to appear in your environment. There more on this topic at this link.

-Jan