To be clear, YES, it IS copyright (or, in some cases, trademark) infrigement. Disney developed their logos and characters, and thus, if you don't have direct permission from them to use their intellectual property, you are breaking copyright (or trademark) law.
There are several sites out that that will print custom tshirts for you (printfection, zazzle, etc.) and they say in their terms that you must only upload designs for which you hold copyright. As such, a good portion of the designs in the DISIGN board are technically illegal.
Beyond that, personal morals come in to play. TECHNICALLY, it's illegal, and TECHNICALLY you can get sued for using intellectual property without permission. After all, it only makes sense for a company to protect its image and usage of the content in which it has invested! Is Disney (or whoever owns the Star Wars properties) ACTUALLY going to come after you for wearing a custom t-shirt? Not likely. Unless you try to sell it, of course. It's up to you whether or not you want to break the law in the first place.