System security and user education won't solve this problem, just because of the law of large numbers.
Let's say conservatively, there are about a billion people using the internet (the true number is higher these days I think, 1B is the number I used to see some years ago). And let's say about 90% of them use Windows. so that's 900 million.
Let's also say that thanks to brilliant virus scanners, spam filters and super education, 99% of users who are sent CryptoLocker DON'T get infected. But unless Windows goes full iPhone and totally bans unapproved software, no defence system will be perfect: let's say 1% of people get whacked.
So that's 0.01*900,000,000 = 9,000,000 (we can assume that after a few years everyone got mailed at least one scam mail with CL attached just because it's free to send)
How many Bitcoin users do you think there are in total, today? 1 million? With a generous definition of "user", perhaps a bit more.
No matter which way I slice these numbers, it seems left unchecked CL could grow at least as fast or faster than Bitcoin itself, to the point that there are more victims than genuine users. And let's face it, 99% of people are not protected.
You say, "real world criminal prosecution" but to prosecute you must find, and I don't see a good way to track these people down. That's kind of the point of exploring this topic.
Here's one question I have yet to see an answer for: Why should Bitcoin, which in many ways resembles, or perhaps can be seen as the next evolutionary step following cash, be held to an entirely different (moral, and legal) standard than cash?
What makes you think cash is held to a different standard? You realise that cash is not 100% fungible, right? If you're found to have forged money, you lose it, even if it's not your fault. And that really sucks because counterfeit currency is not really a rare problem. In the Economist today there is a story saying 1 in 4000 British bank notes are fake!
What's more, if you turn up at a bank with a giant pile of cash, or indeed at a real estate agent, they are expected to treat that carefully and possibly file reports if it seems suspicious.
So cash is not fungible, it's actually very much non-fungible, as anyone who tries to spend large amounts of it at once will discover.
However, I don't think we need to hurt Bitcoin's fungibility to find bad guys and make them prosecutable. See my reply to Piotr for why I think that.
Only have limited time now, I'll re-join the discussion tomorrow. Just two brief remarks:
re: CryptoLocker. The problem with those extrapolations is that they sound about right, maybe they will turn out to be right, but they are very much 'back of the napkin' calculations. I would rather not risk core functionality of Bitcoin to combat a problem that is real, but far from certain to be catastrophic. How exactly is CryptoLocker different from any of the other malware that has been plagueing average users for more than the past decade, causing significant damage, but ultimately not being any threat to the system itself (be it Internet usage in general, online banking, etc.).
re: fungibility. The counterfeit argument is a strawman. Counterfeit money is by definition not real money, so it is not a limitation of fungibility of the actual currency. Also, you will note that I didn't simply say blacklisting coins will limit fungibility, I argued that, because of the ease with which the property of being marked as "used in criminal activity" can spread, it is an actual risk that diminish fungibility enough to make Bitcoin unusable.
There's actually two issues at stake here:
The first is the question of having a way for someone who has paid bitcoins under duress, or had them stolen, to tell others about it, so that those of us who sympathize and want to help (or are required by law to help) can help provide information about the inputs they received, to help track down the original criminal.
In a perfectly voluntary world, I can see such a system being a great idea in theory: The more horrible the crime, the more likely everyone in the chain is willing to cooperate with tracking down the originator. There's still the issue of how such lists are constructed, and how the credibility of the victims is established, but multiple competing lists could keep the process honest (the way spam filter lists work). In the absence of a legal obligation to cooperate, this system works pretty nicely.
The second issue is that we do not live in anything close to a voluntary world, or even one in which governments limit the scope of their authority.
A government would almost immediately spawn their own redlist, and make it a requirement for all businesses to use it.
As you mention, they wouldn't be able to mark "Wikileaks's addresses" directly. What they can do however is mark all addresses as tainted. Every single one. Then the moment I spend any money, the merchant has to report information on my person to the redlist operator (my government). This very quickly removes privacy from the system. Essentially, a redlist system can be expanded by legal authorities to be mostly indistinguishable from CoinValidation's own proposal (by making 'marked' the default state of any funds). The cooperation we've seen between intelligence agencies tells me that we can reasonably assume the results of such a fishing expedition would be shared between most countries.
Nothing we do affects the feasability of this approach from the government. They could mandate it right now, and we can't stop it from coming into existence.
But I would be wary of building the tools you proposed, precisely because that would be making their task easier. Maybe we can actually build it once the threat has subsided due to (insert libertarian utopia of your choice here).
Now I disagree with those who are calling for your head, it seems they're getting a little spooked at how easily such a scheme could come into existence. Let's not shoot the messenger, especially when he's limited himself to a theoretical discussion. There's plenty of people who actually deserve our ire (the CoinValidation thing is positively vile, and they're openly trying to get legislators on their side).