That's the problem with mtgox - there was a divorce of BTC and mtgoxBTC account values, and that divergence, due to poor accounting/theft/whatever ultimately lead to a situation where mtgox is now acting like a fractional reserve bank.
In theory everything at mtgox is fine, UNLESS everyone want their real BTC back now, aka bank run. As long as people just trade between mtgoxBTC or only withdraw in reasonable amounts, it might be ok.
But that won't happen, because of the expectation of full BTC convertibility, people are freaking out and wont cease to freak out.
There are so many parallels between the gold-backed currency and bank runs in that era. Just rumors, or facts of insolvency and inability to produce either specie or gold for deposit values could drive a run and that would be that.
All that annoying bank regulation is there for a reason. It's a good thing we don't have bank runs anymore. Right? RIGHT?
Unless the exchange is taking btc/money from users and investing it somewhere then every btc/$ should be totally within the system, yes?
so you can either withdraw your btc or your $ as the case may be.