I think a lot of people realize that even with the insolvency/seizure risk (and low float rate) it might still better to hold six figures (or more) on an exchange rather than withdrawing and declaring taxes on it. This obviously depends on our cost/benefit analysis.
They can mitigate this risk by buying BTC when the exchange has a cash flow problem or is facing regulatory scrutiny.
One could analyze the cost/benefit ratio this way: One could determine the risk of holding X sum of money on a given exchange (or multiple exchanges) and subtract a conservative amount of risk for proper management (i.e. going all-in to BTC when the risk of seizure/insolvency increases). They could determine the tax liability as well, which is the risk of withdrawal. Compare these risks against each other and one can determine whether it is safer to keep USD on an exchange (with the caveat that it must be converted into BTC, exposing one to exchange rate risk, when the regulatory or cash flow issues present themselves) or to accept the tax liability.
Obviously we can argue all day and night over which strategy has more risk, since some of the risks aren't easily quantifiable. We don't really have the data to asses this (we'd need everything from WMA bitcoin exchange float rates to % recoverable funds to current "bitcoin VIX" or downside move stats tax liabilites in various jurisdictions) but I'm sure there are some people who are starting to realize they can use the exchanges as a 'non-anonymous yet no-tax-liability so long as I don't withdraw' offshore bank account to some degree, holding their fiat there forever and then purchasing, shuffling and anonymizing BTC for purchase OTC or to anonymously/pseudonymously purchase real goods and services without having to declare the income first. I'm not saying this is an invincible strategy, but the law enforcement burden would be enormous, and there are most likely rich folks out there doing exactly what I have described.
EDIT: In case the feds are reading, I don't have anywhere near enough money to even consider myself "a rich person trying to evade taxes on the internet," so please don't waste tax dollars investigating my broke a$$. I'm merely speculating on what these bastards are doing with the bitcoin price.
Good thinking. We the rich, prefer to invest cold-bloodedly, and assess the risks in percentages. In my case, for example, selling bitcoins from my long-term personal holding (which has never been dipped into), would result in 24% tax of the amount sold (since it is practically all profit, measured in fiat).
Even if there is 50% probability of Gox going belly up this year, and in 20% probability this will lead to the total loss of BTC holdings there, the estimated loss is only 10% (50%*20%). Much less than the loss to taxman by selling the coins.
Government has confiscated physical gold and silver from me, my companies and my customers in 2008. It is not that all the value confiscated will be indefinitely kept by the government, because it is illegal and unpractical for them to do so. The risk premium (discount applied when my customers bought silver in gov custody, to be delivered when it is returned to me) was 10% during the time. The metals were held for 7.5 months, returned in full after the investigation and no charges were filed.
The feds are threatening the bitcoin holders with unlawfully long process times. They don't have the authority to take and keep your bitcoins without due process, not any more from Mt.Gox than your personal holdings.