I am not a lawyer, but I have worked with some in the tech industry. You should consult with one if you are going to produce something that is SO similar that it cannot be distinguished from the original.
From
http://www.gillhams.com/dict/copyright.cfm (I believe this UK based firm operates globally)
"Copyright is one of the central branches of the law of intellectual property. The law of copyright gives an owner the exclusive right to prevent reproduction of certain works by others. Copyright provides owners with some element of control over the exploitation of their works. Copyright protection applies automatically from the moment the work comes into existence and subsists usually for at least 50 years. Copyright does not extend to ideas, or schemes, or systems or methods; it is confined to their expression - that is the form in which they appear either on a page or the form in which they are otherwise recorded."
For US specific information:
http://www.copyright.gov/circs/circ1.html#wciI can't recall the exact formula for factoring this, but I believe that you would have to have 70% original material in the object in question to have it ranked as an original work.
Now, if you see an object or a picture of an object, and model it on your own, using your own tools, and your own skills, it is in fact a NEW ORIGINAL. As long as it is not an exact copy of the original, it is in itself, unique. Otherwise, every model of every real life object could be considered a breach of copyright.
If you create for instance, a starship that consists of a saucer, a series of tubes, and arrange them in such away that they resemble StarTrek-style ships, well then you are moving into trademark territory. If you decide "Hey, I'm going to call this tube and saucer ship ENTERPRISE" then you are cooking up a nice little litigation stew.
In my opinion (and it is only an opinion) it is fair use to take inspiration from the objects, pictures and ideas that you find wherever you find them. Nintendo does not have exclusive rights to all likenesses of a fat little Italian guy with a moustache. Just make him somewhat unique, don't call him Mario, and don't mimic the gameplay and events that Mario has gone through, and you can pretty much do what you will with the idea. IDEAS are not copyrighted. They can be pateneted, though, if sufficiently documented.
Now, if you take a model, re-skin it, and call it your own, you again have a problem. Take the model apart, make your own structure to create the model, generate a skin for it on your own, and you have a new model. It's shape and construction are not the same.
Copyright is a sticky, tricky and confusing topic. I can't stress enough the importance of giving credit where credit is due, even if for inspiration, and for creating your own models. Take lessons where ever you find them, inspiration from all who inspire, and money from anyone who dares to do anything similar!
Good links:
http://www.ncte.org/pubs/chron/highlights%5C122872.htmhttp://www.lawmart.com/searches/difference.htmJust my 2¢ (Wish I could trademark that little penny symbol...)