Their word that they rewrote the code for it from scratch, closed down all access they could, and are now storing most coins in offline storage. Also them putting limits on all withdrawals, requiring some type of ID for anyone wishing to withdraw substantial funds, and being the first to use two factor authentication. Plus the part about them still being the top exchange by volume by far, and yet not being hacked since that last incident almost 9 months ago. Also, I wouldn't be surprised if a lot of the common sense ideas everyone uses now (cold storage, withdrawal limits, two factor option) were things people didn't care about until MtGox incident, and which they got from MtGox since then. I wouldn't be surprised if Bitcoinica came up with new security procedures that everyone else six months from now would look back on as a no-brainer, and at the very least this would emphasize the urgency of implementing multi-sig security, whereas without it people would have greeted the change with a "meh." In fact, I'd go as far as to say we were about due for another major security breach to get people to learn more about or invent better security measures. The more that happens during Bitcoin's development stage the better.