Okay, I officially loved this :). Short and awesome, and Draco's voice was made of pure win.
"Potter," Draco says slowly, his grin slowly morphing into an incredulous smile, "are you getting hard?" He grinds his hips against Harry's, slowly and precisely, and Harry's mouth falls open at the deliciousness of the feeling of a hardening cock against his own, even through his trousers. "You pervert, you are!"
Aaaaand I might be a little bit in love with him for this scene :D. Omigod, yay for a totally natural and fun interaction!