Copyright law is a pretty interesting thing. And we can really thank Disney for making it into such a big issue that almost everyone is afraid to do anything now.

I agree with everyone that have said that it would probably be considered "preparing a derivative work," however some more things can come into play -- if the work is intended as art and will only sold once (each picture is an individual one, no copies would ever be made) it could be argued that you don't need the original permission for various reasons. But it would be much safer to get it first than it would be to make the items and then try.

Also, copyright law greatly favors large companies, and I believe (please feel free to correct me if I'm wrong) that you can copyright things that you use in a product at pretty much any time -- so even if the slogan of "Shoot the Core" isn't copyrighted now, if Konami wanted to sue you on it they could based on the fact that they might have copyrighted it in the future. I don't remember the specifics, but there are some strange things involved with stuff like that.

Lastly, the reason why a place like CafePress would be very wary of pressing shirts that probably will avoid the copyright issue is twofold -- the first issue is that if anyone gets sued, it won't be you for making the design -- it would be CafePress for selling it. CafePress has a lot more to lose if they get sued than you do for only selling the shirt.