Total disaster, never happens in real world, not virtual one. Except for all the times when 'real world' currencies undergo devaluations, revaluations, forced exchanges, just plain old inflation, all the things that lead to currencies collapsing. I mean name me a paper currency that lasted longer than 80 years on this planet without a major restructuring, without collapsing?
I agree that the dangers of bitcoin forking have been overstated, and are something of a manufactured drama - technical problems like this are not very difficult to surmount. The real problem with Bitcoin for me is that the system is not transparent, and nothing backs the currency (unlike those fiat ones you mention above).
A currency is a token of trust (trust that others will value it the same amount), and that's a fragile thing.
Bitcoin is currently a small curiosity, it's only just becoming big enough to attract the interest of the real sharks, and I'm not convinced the creators have the resources, motivation or interest to keep the currency fair and secure once serious money becomes involved. Many of the exchanges are still pitifully insecure (run on VPSs !), the infrastructure is not well managed (witness problem above), and the creators probably never expected it to take off or really thought through the implications. Once there is serious money involved, lots of people are going to want to change the rules. If Bitcoin becomes popular it will be easy to coopt, devalue, and tax until it is just another currency, probably tied to a particular corporation or government. There's absolutely nothing you can do about that as a user of bitcoin. If the developers decided to change the direction of the currency you have your life savings in, devalue it, create a new block chain, you don't even have a vote on the matter.
Currently, if the government of your country or anyone else with the power to control the flow of bitcoins decide it should become valueless for you, or illegal, that can easily happen, if someone corners a significant supply of coins, they can manipulate the market (this is probably already happening as there are ZERO controls in place to stop it), if the public panics due to misinformation or rumour in such an illiquid market there is nothing to stop huge swings in value, and if a government decides to coopt the currency, shut down exchanges and change the rules by fiat, no-one is going to be able to trade in it and interest will evaporate. I see that as the largest problem with bitcoin by far - there are no backers putting up their own goods, no-one to trust, and no way to ensure that others continue to play by the same rules as they used to. It's certainly very appealing to utopian crypto-anarchists, but of limited interest to anyone who wants to store value or exchange it, given that it has the disadvantages of cash (anonymous, fungible) with none of the upsides (backed by a sovereign government, relatively stable, regulated to a greater or lesser extent, insurable etc), and a few downsides of its own (massively fluctuating value, built-in deflation, early-adopters privileged).
Because it is untraceable, and not guaranteed by law, it's of no interest to the majority of people who use currencies to store and transfer value and receive payment. I *want* my transactions to be traceable, so that I can prove to gov. and counter-parties that I have fulfilled my part of a bargain, made a payment, and should receive goods or services in return. If I don't want a transaction to be traceable (very rare, but conceivable), I'd use barter or some kind, but a currency outwith the control of government holds little interest for me, *precisely because* it is outwith the control of all the rules of society I value. Those who've had their valuable bits stolen from some VPS have no come-back using bitcoin, and no way to find a thief or enforce punishment - I'd demand far better than that for any currency I put trust in.