It should be blatantly clear to all that MTGOX doesn't have the cash flow to operate in too large a capacity as an exchange. Sure they can handle small withdrawals to keep the illusion up but choke on anything large. I highly suspect they are operating on Fractional BTC reserves also and if people started making huge BTC withdrawals, we would start to see similar actions there.
~BCX~
It's probably more complicated than that, if they wanted to increase their reserves, they would just increase the fees until it balanced.