Chris Breen has posted his instructions on how to access the iPod mini’s Diagnostic mode.
How to access iPod mini Diagnostic mode
“The Diagnostic screen has changed since I mashed one of the beta units at Expo. To access that screen, reset the mini by pressing and holding Menu and the Select button for about six seconds. When you see the Apple logo, press and hold Previous and the Select button until you see a reversed Apple logo. Let go and you’ll see these entries:”