[Solmu]

Some people do it, most don't. As a customer it really, really annoys me (to the point that it will almost always stop me from buying a shirt if I notice in advance).

In my opinion it almost always gets in the way of the design.

If you really want to put a copyright notice on the shirt I would recommend putting it inside - either screenprint your own label, or get a woven one, with a copyright notice in it.

Not only do you not need to register a work to be protected by copyright (only to prove when you created it), it is my understanding you don't even need to display a copyright notice. Things are only in the public domain if they are explicitly released into it. That said, I am not a lawyer and could be wrong - so I'd look into that if it would affect you in any way.

Copyright notices on websites are harmless anyway, so I'd always include that in the footer (and a few other places, like the FAQ or contact page).

 

[Solmu]

But; what happens if somebody uses your design and changes it just a little bit? I'm afraid copyright will not be valid anymore then ...

This is a common myth, and you can put those fears aside.

Copyright law is incredibly clear on this issue - derivitive works are not allowed.

Parody is protected (under the US constitution as I understand it - I believe it's an extension of the right to free speech) as a means of political comment, so there is some leeway. But you would have to prove your design was a parody and had artistic or political merit - harder to do in a commercial context.

Granted this won't stop people doing it because they think they can get away with it (or are under the gross misconception that it's legal), but hey... you can always sue them