glass - best bet is probably to offer them some sort of agreement that commits you to not divulging the details of your design to anyone else. You retain the copyright, but they retain exclusive use of the end product subject to your license agreement.
Your concern is justified. A colleague of mine designed a large building for a client. Got a call a couple years later from a building official in a different county with questions about the building. Thinking it was strange, he asked for a copy of the drawings. The original client had put a new address on the title block and resubmitted it to build the same building again. He sent a fee for the second use to the client with an ultimatum - deliver payment by COB next day or the next call would be from his attorney for copyright infringement and violation of contract. He got the check.
For the mechanical guys - I agree. My last employer hired mostly mechanical and electrical engineers to design large scale industrial equipment. Most of the work was proprietary, and so we insisted on owning everything. It typically works differently in the Civil/Structural world. I guess it has to do with our lack of prototyping. With a lot of the equipment we were getting, it would be designed, delivered, and run through a series of tests to confirm that it works. Once it's done, that's it. We weren't manufacturing more of them, but we owned the rights to it (we essentially funded the testing process) and prevented it from being used for a competitor. In structural design, though, we design it and it gets built. There's very little testing to make sure it "works" under design conditions, because design conditions are relatively extreme and often result in damage anyway. Our clients are often another step in the design chain or a contractor, so there's a very real possibility that they will go on to make another one and, if they do so, our liability increases as there's a chance of a mistake in the design being replicated without notice.
