This is very much based on your list of illegal stuff. If you go down that list to the boring stuff, the rule that government intervention increases demand isn't true.
60w light bulbs were made illegal in the EU a few years ago - we must now use low energy light bulbs which cost 10 times as much. But there are no dodgy light bulb salesmen hiding in doorways trying to sell me illegal light bulbs.
Leaded petrol (gasoline) was made illegal 20 years ago, yet, there was never a demand for this old fuel from hardcore drivers, not happy with the alternative.
Even cigarettes which are being made more and more illegal through harder ways to buy them is also suffering from a lowering of demand. Latest usage figures are touching 20% of the population, down from 50% 40 years ago.
Its only when the item being made illegal doesn't have an alternative which is good enough for the public that government's find that there is a black market formed. And in bitcoins case, I don't think that is going to happen. While its cool to be able to use bitcoins to cut out government, I don't know how big a market there is for that exclusive use - I don't think its so big that people are willing to go against big government to get their hit!
Although I could be wrong
Yes, this is key. I do have high hopes that governments will not attempt to shut down the bitcoin market (not that they could,
entirely, of course), and the relatively non-threatening nature of FinCEN's scheme supports that. But personally I am cautious, as I have in the past lost considerable money to government seizure in the grey market.
There is currently little demand for bitcoin as currency. We are nowhere near the critical levels of adoption that would be required to stave off massive downward price movement in the case of being targeted by major governments.
If this were to happen, speculators -- virtually the only thing that is massively propping up bitcoin prices -- will dump en masse. They will move on to much safer investments.
In the case of online poker, US consumers clearly decided in favor of brick-and-mortar casinos and specifically legal forms of gambling. What will demand look like if new users begin to get the impression that dealing in bitcoin is "illegal"? If regulatory schemes come about that are too costly or legally uncertain, venture capital dries up.
Not trying to troll... this is a scenario that should be considered. I lean bullish longterm, but I consider bitcoin quite risky (this is only one scenario that could crush it). And those perma-bulls who have pie-in-the-sky ideas about what regulation will look like need a reality check. This will make or break bitcoin.
I find that we often talk here as if bitcoin has already achieved mass adoption. It hasn't. We are a very long way from that. The new money coming in -- how long it will sustain we will see -- IMO is not from bitcoin users, but by and large from speculators. That is not sustainable.