bitcoin7.com has just this little text that they've been hacked and are slowly returning coins and money back to people.
What if MtGox suddenly has that and it got hacked big and is shutting down. What would happen to bitcoins?
Having an exchange responsible for 90% of bitcoin trading suddenly fold would have an enormous impact on bitcoin. Many people would lose a lot of money. Confidence would be shattered. Sure, another exchange could rise up and fill the void, but would anyone want to use it? It's also much easier said than done to 'just open another exchange'.
Many people would be asking themselves 'what's the difference between an exchange getting hacked, and the site's operators stealing all the bitcoins and declaring a shutdown due to hacking?'.
MtGox has been taken down once. I hope they have 24/7 active monitoring and regular external audits.