I'm the founder of Bitcoinica.

The confusion is simple: Bitcoinica is not (yet) a place for you to change your Bitcoins to USD. It's a trading platform for investors and speculators to gain profits from the trade. Bitcoinica is a professional tool.

You can't go to a forex broker and change your dollar to euro because you want to travel.

Bitcoinica uses net accounting system, all the profits and losses will only affect your USD account, but you can use both currencies as margin to trade. You're not trading BTC at Bitcoinica, instead, you're trading BTCUSD, which is a contract based on the ratio of values of these two currencies.

Since your USD account is negative, you can't withdraw. But I have settled your account, by using part of your BTC balance to fill the gap in your USD. Now you can withdraw BTC to your wallet!

I'm sorry that Bitcoinica is not for you at this moment, but we're developing features for people who just want to exchange Bitcoins too.

Currently we have more than 800 registered users, and most of them like Bitcoinica very much. (Most likely because they know what they're doing.)

We will keep you updated.