We had a similar problem. We made a parody T-shirt that showed a Lego-like parody of an anime figure from an anime called Akira, basically showing you how to build a Lego man and motorcycle. It was a joke, but Lego came at us. Obviously we're not going to get into a major legal conflict over one T-shirt, but we're getting our revenge. I told the lawyer for Lego that if they forced us to remove the shirt from our line, we'd make a George Bush "blockhead" Lego parody T-shirt that was, of course, very different from their designs. They come out in February ^_^ See them here:
http://www.jlist.com/jhome/p001.jpg