I've seen a pit with it's ears pierced before. I'd put it in the same category as docking tails (for cosmetic reasons) or clipping ears. It isn't something that I would do, but I don't think it's any crueler then that. In fact the piercing is probably nicer then clipping the ears.

*edit* I was just thinking, maybe piercing isn't more humane then clipping ears. You'd have to have a very calm and only dog for that to stay safe. If Dusty or Roxy's ears were pierced, they'd rip them out of eachother playing within minutes.