The bid will be put automagically by Mt. Gox whenever someone do this kid of trading. The price isn't going to fall below that bid until everything has been bought at that price by someone.
That's not foolproof. If there is a big bubble and it bursts, there could be no buyers
ever at the liquidation price. But that's a commercial risk that MtGox will need to decide.
I will be
very surprised if we ever see margin trading at big ratios like 100 to 1.
Personally, I wouldn't trade on margin because I like to buy after prices drop, rather than being forced to sell at the low price. Sure, the successful margin trader more than makes up for it when prices rise, but not everyone is successful.
For those who don't trade on margin, the existence of margin trading tends to exaggerate the peaks and troughs of the market. When prices are rising, it greatly increases the number of BTC that can be bought, causing the price to rise further. When prices are falling fast enough to force liquidations, it greatly increases the number of BTC that are being sold, which depresses the price further. I don't mind this, because it gives me more opportunity to buy very low and sell very high.