I spoke to a manager at Play.com, and I have to say their customer service has been excellent thus far (obviously not the unfulfilled order part) and he was very helpful. He explained to me that it never should've been put back up on the site, as the stock was actually stock they were waiting for to fulfill existing orders who hadn't yet received their copies. He said that none of them should've been despatched, but it obviously looks like some have slipped through the cracks. I suspect their will be a fair few very angry collector's contacting Play over the next few days, as by the sounds of it they can't get hold of any more copies and they have given copies, which should've been reserved, to people who didn't order them originally.
The same with IM2, apparently they've received 70+ complaints and they've spent most of the day trying to resolve the issue.
The proverbial **** has well and truly hit the fan at Play HQ!