Well, if it was coming at it at its blunt side, i dont think it would do that much harm. If it came at you on its edged side, owie. I beleive a hard punch travels at that speed.
One of my father's friends was going to smash a brick against a mailbox when he was a teen. He stuck his hand out the car window ready to let the brick sail right into the mailbox, but he let go too late, and the hand smashed right into the mailbox -- with the brick in the hand -- while going around 70 mph.