I just got this email from Amazon.co.uk:
"We regret to report that the release of the following item has been cancelled:
Colin Morgan, et al "Merlin - Series 1 - Complete [DVD] [2008]"
This item has now been cancelled from your order #XXXXXXXXXXX and we can confirm that you have not been charged for it."
The DVD seems to still be for sale at
Play.com and
Amazon.co.uk, though and they changed the date back from 23rd Nov to 5th October.
Edit: I have no idea if this is a bug, but I've been allowed to pre-order it again from Amazon.co.uk
Edit2: Amazon confirms it was a bug: "We are currently investigating the possibility that this order cancellation and related email may have been the result of a system error which has now been fixed."