Well, fortunes can be made at a roulette table too. And watching the price investing in bitcoins is on par with roulette.

Lost AND made. Every loss is someone's gain. :)

Not so with assets. If you value your Bitcoin at $1000 today and tomorrow it is worth $800, you have lost money and no one gained it.

You can short on various exchanges, so as the market price drops, lots of people make a lot of money.

Even without shorting, as market price moves downward, it is only a loss for those holding. Those who got out at $950, and bought back back in at $800 have increased their holdings - as price fluctuates in the future, they increase their leverage.

Finally, you are focusing on fiat which is a tunnel-visioned approach. When BTC/USD drops, for those already in fiat it gives them more buying power. This is a gain, as yesterday they only had 1 BTC worth of fiat, and now they have 1.2BTC worth of fiat.

So it all depends on perspective, and you are right in your view. However there are many ways of looking at things, and my view isn't necessarily right either. I would say they are all valid examples though, of different ways of looking at it. Much harder to say which is "right".

Well BTC went down 20%, but since 80% of my portfolio is non-BTC, my BTC equivalent went up 20%!

Reality is you just lost quite a bit of money, but it makes it sound positive. Not that you automatically make these mistakes, but it is trivial to do that when letting your other currency float.

I am not sure if this statement is correct or is likely in grey area. The person who sold it to you has gained $200 if they choose to buy now.

I don't like to let the non-fiat float, it makes it possible to say anything.

"Your BTC is worth 80% of what it was before, but since you are only 20% invested in BTC your BTC value has gone up 20%!"

how do you sell or buy a mtgox bitcoin?

or mtgox-otc if you don't want to pay fees to a third party.