"Um, son, you're going to have to come with me"
"What? What for"
"Mr. Miyamoto would like to have a word with you"
"What the fux?!"
*Returns an hour later with a broken DS and crushed flashcart, not to mention Miyamoto personally kicked you in the balls"
You try and fuck Nintendo, and they'll fuck you.
--
They'll probably disqualify you from the tournament.
I've taken my flashcart with me to Toys R Us in the summer of '05 to get Animal Crossing items, they didn't do anything...they couldn't.
It's not illegal unless you've got Nintendo DS ROMs on it (not sure if it's legal if you really do own a copy of the ROM on your flashcart). It's legal to play music, videos, play homebrew, etc.