Wow, scary link, I had no idea that it could be IB. I get a wrinkled one very ocassionally from one hen, but I think it has more to do with being soft-shelled, as they are definitely softer than a normal egg.
I have just found a wrinkled egg very much like the one pictured on the lik but there are no other symptoms of IB. I think they are moulting and its hot too. I have been giving them cat food recently to help them with the moult. Could it also be a lack of minerals needed to form the shell?