I've just found this article regarding
copyright and artwork
Even though it doesn't completely answer your question, it seems that your employers could have a case.
It does seem very harsh for your employer to take such a stance. I can understand that they wouldn't want you to show work that hasn't been signed off or is still in development stage but I've never heard of having to remove the clients details from artwork before adding it to your portfolio.
It would be interesting to hear if anyone knows the definitive legal answer to this?