This should be going into the SU wish list. I'd like to be able to lock a model somehow, have it only respond for viewing.
Gaieus, you're correct, passwords can be cracked, but you need not make it easy to do. Banks use 128 bit encryption (AFAIK) for online banking, so I would assume that that level of encryption would be the starting point.
Personally, I don't believe in releasing the CAD / 3D data to a client. I've done it (once or twice) before, but I put up a good fight and won't release it unless I'm well compensated for the data.