The short selling option at bitcoinica is actually helping avoiding bigger sell offs, as short sellers MUST eventually buy back to avoid getting wiped out.
Now, with bitcoinica down, no one has to buy (=cover shorts) any more, and selling pressure could mount.
That makes no sense. To short on Bitcoinica you need to have some money in the system. While in a big swing your account could go negative, most shorties would not have a negative balance. They have lost funds that if they want to recover they would need to buy more bitcoins. They may choose not to, but shorting on bitcoinica could only drive down investor sentiment but it can not actually create more bitcoins. In the stock market, because of the three day rule, people can actually create more stock for three days.
I do believe this incident will lower the price of bitcoin, mostly due to sentiment and due to the thief potentially selling. But it will not cause a price drop due to the lack of short buybacks.