Earlier quoted context omitted.
How would MtGox effectively "create BTC"? There is a difference between moving coins around in a database and creating new coins. All coins are deposited from users. The only way BTC would be "created" in MtGox is if MtGox was mining on their own. Coins need to be "mined" to be created - so MtGox couldn't even loan coins they didn't have.
MtGox could effectively "create" BTC by simply selling more than they actually had. This would of course only work as long as people were actually withdrawing less than MtGox had available to them. Until you attempt to withdraw the BTC from the exchange, you wouldn't necessarily actually know that they in fact exist.
I guess your theory will be tested when a large majority pulls their money from Mt.Gox...that is if that happens...we will all know in the next few days.