Ok, whereas amne's basic problem seems to be fixed (so I'm renaming the title) 
I just got a bugreport by Sven Joachim which is kind of similiar:

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Rebooting grml 1.1 (the full i386 image) with Ctrl-Alt-Del fails rather
badly.  After ejecting the CD-ROM, closing the tray and pressing
<return>, the kernel spits out lots of error messages to the console
regarding I/O errors:

/dev/hdc: tray open
end_request: I/O error, /dev/hdc, sector 114852
loop0: read i/o error, sector 84182

The last messages tells that executing /sbin/reboot failed and that no
more processes are left in this runlevel, at which point only the
power button helps. :-(

Sorry for not providing the exact messages, but I don't know how to
paste them from Qemu.  I have attached a screenshot, though.

Besides on Qemu with the ISO image, I've seen that today on two
different real machines with the CD-R burnt from that image.  And yes,
I have checked the md5sum of the image.
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