Although it could have some impact, your money isn't "locked" with MtGox due to being able to transfer it to another exchange as Bitcoins, and cash it out as another currency there.
Most on Mt. Gox don't have secondary accounts, and this is more likely for those who are looking to make short term $ (and who tend to panic sell). For the various exchanges they could move to, what's the turnover time to be able to trade and more importantly, make USD withdrawals? Will the other exchanges be ready for this? How exacerbated could this be if there's a price drop?
A comment based entirely on speculation. Do you know for a fact that most people on Mt Gox don't already have another account elsewhere or can't figure out how to create one?
