Key combinations that trigger special functions in the bootloader or watchdog or whatever:

iPod 1-3G
    
    - Reboot:  hold Play/Pause + Menu
    - Diagnostic menu:  hold Rewind + FastForward + Center   (use rewind/fastfoward keys to navigate the menu)

iPod 4G+  (includes Photo, Nano, Video, and Mini):

    - Reboot:  hold Menu + Center
    - Disk mode:  hold Play + Center during bootup
    - Sleep:  hold Play/Pause       (doesn't go through the bootup process, so it probably keeps things in RAM, but the touch part of the wheel stops working)
                                            (though the alarm clock continues to work in this mode)
    - Diagnostic menu:  hold Rewind + Center during bootup   (use rewind/fastfoward keys to navigate the menu)