I believe it's velcro... If you put Vader's TIE fighter on a shirt with just a glued plate I think it would rip it off...
Alright, it definitely isn't a top ThinkGeek quality item (they have a pair of his and hers shirts that light up based on how close one shirt is to the other...)
I think I'll pass.
EDIT: I just saw the "believe" in that sentence. I know that it would fall off in reality, but that was done with the shirt out on the floor. Anyone on this board own this?