I recommend doing it on trains and buses. Sound files are nice too, you hear if they play it..I'll have to start writing notes on the computers at the school bookstore.
On a side note, I tried to bluetooth a picture to someone in class today. She had her mac running and I tried to send it from my phone. Not sure if she got it, but that'd be funny if she did! It just said "Hi!" with pens sitting on a desk. I'm going to just start sending random stuff to people that leave their bluetooth phones on.