Right, that's why merchants generally won't do that, nor should they. They can just sell the Bitcoins at the present rate and *still* benefit from their future value because they are selling that future value for its full value. When you sell a Bitcoin today, one of the things you are selling is the right to hold that Bitcoin and benefit from its appreciation in value. The price you get will include the value of that.
If a merchant wants to sell his product for $100 and accepts BTC at the spot exchange rage equaling $100 and then immediately liquidates the BTC to that $100 then the merchant would be avoiding all speculation and any risk of loss or potential for profit from the change in exchange rates. Such a merchant would thus gain nothing from the appreciation in value because he dose not hold the BTC for any appreciable amount of time, without holding over time you can't enjoy the benefits of deflation.
That's not what I said. I said if the deflation is predictable and reliable, it will be reflected in the current price. If it's not, it won't induce hoarding. You can't have it both ways.
What exactly do you mean by reflected? At first I though you meant something like "an expected future price of X will cause price to rise FULLY to X" but perhaps you just meant it will rise some amount towards X but not entirely to X. Obviously 'predictable and reliable' has a lot to do with how strong that rise is going to be, the less people predict the future increase in the commodities value or if they think it is unreliable for any reason they will be less inclined to buy the commodity for the purpose of selling it later to reap a profit, aka speculating. In an economy of actors endowed with perfect future knowledge their might indeed be price rises all the way up to the future price but in the real world no consensus on the future could be that perfect. This is the point I'm making, that their will always be a gap between present prices and future prices, we can never arrive at a point when the future has been so fully accounted for in the present that prices stop changing.
Also I think your double negative "If it's not, it won't induce hoarding." has one too many negatives. Certainly if present prices are lower then future expectations then people will hoard/speculate in the commodity. If present prices match expected future prices then speculation would cease.
But that's the beautiful thing, you don't have to. You can get the benefit of the fact that computers get cheaper over time without having to delay your purchase. The price of computers *today* already reflects the fact that they get cheaper over time. The prices would be higher if not for that. The price of oil today reflects the expectation that it will be more expensive in the future. Google "speculators raise price of oil" for a thousand articles explaining how present prices come to reflect expected future changes in value.
I think both your computer and oil examples ignore a major factor, the producers of both commodities are not speculating in them. A computer manufacturer needs to sell it's computers so it can make a revenue in the present and spend that money on making more computers, rinse and repeat and make a small margin each time which adds up to enough to cover over-head costs and then leave a decent profit at the end. Oil producing nations generally need to make money to run their welfare-states which hold Arab-spring like revolutions at bay (temporarily). The fact that computers made today will become obsolete in the future is not really deflation, it is a product perish-ability issue. Just as a Farmer who has grown a bushel of apples needs to sell them before they go bad the modern computer needs to be sold before it becomes obsolete. The Farmer would still need to sell fresh apples urgently (which would keep prices down) even if he knows the price of apples will be high next year, he can't speculate in a perishable commodity. Together the need to turn over inventory and technological obsolescence (or sometimes just fickle consumer tastes) force producers to sell their newly produced goods at the present market price, not at some price that reflects what the good would sell for in the future.
The Oil speculator thing is something I'm quite familiar with and a piece of conventional wisdom, but I'm sure you would be the first to admit that 'conventional wisdom' is an oxymoron. The price of oil is actually rather resistant to manipulation by speculators because the world storage capacity for oil is very modest at only a 3 months worth of imports. Oil is sold on a futures market and a futures contract gives one the right to buy a commodity at a certain price in the future from a certain producer, if the market is glutted at that future date then the contract is worthless because anyone can go into the market and buy for less then that agreed price. So a wide belief that a commodity is going to be in shortage in the future can indeed cause future contracts to be sold at very high prices, the shortage must actually materializes for the price to rise, it can't be raised by the speculators in the futures market alone. Only when someone buys and removes supply from the market thus creating a shortage can speculation produce a rise in prices (this actually happened in the Onion market once when all Onions futures were bought and the Onions stockpiled in Chicago it caused Congress to ban all futures contracts in Onions, the only commodity future banned in the US today). Oils thin storage capacity means that buying and storing oil to try to drive the price up is rather self defeating, because full storage is the main indicator to the market of a future drop in price. During the witch-hunt directed at 'oil speculators' in the 2008-2009 period oil inventories were continuing to fall, not rise, thus the whole thing was completely baseless and served only to distract people from the far scarier problem of Peak-oil and the failure of the political classes to reduce dependence on foreign oil.
Obviously BTC isn't perishable and it's ridiculously easy to store so price can go (as people here love to say) TO DA MOON in a speculative bubble. But were talking about how the normal economy works here with computers and oils and such, BTC is a completely different and highly abnormal commodity and your applying it's strange qualities to real things and thinking this is how real things work too.