Comment 17 for bug 19634

ThinkWiki has some more information on this:
http://thinkwiki.org/wiki/Predesktop_Area
which links to
http://thinkwiki.org/wiki/Hidden_Protected_Area
http://thinkwiki.org/wiki/Rescue_and_Recovery

The basic gist is that there's a hidden part of the hard drive (at the end) that can be booted into which contains backups and diagnostics.

The tricky part is that the implementation has actually changed over time.

Here's a summary of what I've been able to figure out.

On older ThinkPads, this is implemented with the BIOS hiding that part of the hard drive (by reporting a smaller hard drive to the OS). In this case, the MBR contains a special ThinkPad bootloader which monitors the blue button. If the blue button is not pressed, it boots the first partition on the drive regardles of active bit. If the blue button is pressed, it does something magical to boot into the hidden part of the hard drive.

On newer ThinkPads, the hidden part of the hard drive is a standard hidden partition, and the ThinkPad bootloader unhides it and boots into it when the button is pressed. In this case, it is possible to have GRUB replace the ThinkPad bootloader.

Installing any bootloader into the MBR that is not the ThinkPad bootloader will result in difficulty booting into the predesktop area. Depending on your model, you may be able to configure grub to do the same thing.