Comment 26 for bug 560839

A simpler workaround which worked for me but might not work in all cases or might not be available is to use the "multiboot" entry in the boot menu; in maverick, you would have to re-enable it in /etc/grub.d/20_memtest86+ and run update-grub.

The reported reason why this entry is commented out in Ubuntu by default is:
    - Drop the multiboot image from the GRUB menu for now, since it's
      experimental and has known problems detecting all memory on some
      systems at the moment.

so double-check that you see all your memory when using the multiboot version.