See my sig. Then notice everybody's lack of balls to actually do it.
It would depend on the bet you'd made. But for the "normal" bets, I'd owe you 10 coins (plus your initial bet, obviously).
It isn't exactly shorting, its closer to a binary option. The only way I could do shorting though is if I somehow had some way of causing a margin call, and even that would be hard considering the insane volatility currently associated with bitcoin. I suppose I could do a "Contract For Difference" with the potential for margin calls if you really wanted, though. I figured people probably wouldn't like it as much because it isn't quite as "exciting" and requires them to trust me with more money to get the same risk/reward.