Won't hurt the tank, might be a little unsightly, but it will get covered with bacteria and be just like any piece of gravel.
Says so right here:
http://joejaworski.wordpress.com/200...k-need-carbon/
Spilled carbon causes harm to the aquarium False. Carbon granules that are accidentally spilled into the aquarium will quickly become saturated with bacteria slime, having the same biological effects as a grain of gravel. It may look ugly, but it is totally harmless.