I would say a crash is almost guaranteed, the question is when and at what price. A few weeks ago people were having this conversation when it peaked $30.
It will probably crash. Obviously it has had greater accessibility and hype lately. Coupled with profitable mining becoming impossible for most people riding the boom looks more and more attractive. At some point in the future the buying volume will taper off due to less new speculators getting into the game.
If BTC had anything actually underpinning its value we could work out the ratio of how overvalued it is. Unfortunately it doesn't - even the fact that real transactions are made with BTC doesn't change anything, as the value is so elastic that a T-shirt selling for 1 BTC 6 months ago would sell for a fraction of the BTC now.
If the currency is stable now, what exactly is making it stable? Where does the equilibrium come from? Isn't it unstable by definition anyway since it's rising uncontrollably?
"Coupled with profitable mining becoming impossible"
Well it is incorrect to state that. Obviously, as the exchange rate increases, mining becomes more and more profitable. In fact, as of today ($50/BTC, difficulty factor around 4.9M, 25BTC/block) mining has never been so profitable in over a year(!) since January 2011 ($6/BTC, diff=1.1M, 50BTC/block)!
There are still many people holding large amounts of bitcoin (because they mined them in the early days) and just one of them selling everything would cause a pretty large downswing.
That is awesome.
If you follow the chain of transactions from 12RYJjGk22NiNbhTHffCrCso4XgVzet5Eh (biggest wallet). It all ended up in a MtGox account...
For such large amounts, you can't simply calculate the number of bitcoins * price per bitcoin, since the act of selling such large amounts would itself affect the price.
This is true, of course (and a pet peeve of mine when people naively calculate e.g. how much it would cost to replace all coal-fired power plants in the world with solar). However, these days MtGox's average daily volume is in the $1m range. I think you could sell $1m BTC over a couple of weeks without anyone even noticing.
Same applies to a lot of wealthy people in shares. If they sold all their shares, they'd make the share price plummet and they wouldn't get anywhere as much as you initially quoted.
As volume picks up (market depth has nearly tripled in 2 months) this is a lesser problem.
No, we know that there is some fuzzy nebulous number of bitcoin millionaires which may be centered around 25 - more based on how many have coins split across many addresses, and less based on the known fact that many earlier miners didn't store their coins safely (because it was just an interesting crypto prototype they were playing with) and many tens of thousands of coins have been lost. (That Polish exchange running on Amazon without backups took out, IIRC, something like 10k all on its own.)