I never said my english was perfect, but surely my spanish is *far* better than yours, you grammar nazi.
negative balance has nothing to do with leverage, if liquidity is inexistant at liquidation time, you also can go
negative with 1:1, 1:100 or whatever.
Naomi is completely correct that the opportunity to leverage investments on Bitcoinica makes it possible to lose more than your initial investment.
When you borrow to buy assets, this is called leverage.
If you take a short position you have leverage. You are borrowing the currency you are shorting and the value of your debt can grow to exceed the value of your holdings. This is when forced liquidation occurs.
If you take a long position. You don't need to have leverage. You can buy the currency you are investing in. If you do this, you take on no debt and there is no way for forced liquidation to occur.
Leverage is necessary for forced liquidation to occur. To end up with a negative balance, you need to take on leverage.
If you are heavily leveraged up, then, fixing your initial investment and the degree of illiquidity when forced liquidation occurs, the size of your negative balance will be proportional to your leverage, i.e. if you leverage up 10:1 you will end up with a negative balance ten times as large.