WOW is taking a page out of Eve's playbook and making game time purchaseable for in game money. Players will be able to buy subscription time in units of 1 month and sell them on a special part of the AH with no fees for gold. Blizzard has clearly thought a lot about this and has gone about it in a cautious and sensible fashion which I approve of greatly.
One of the big concerns people have is that buying time would involve a lot of the usual AH shenanigans. That is, people would buy up huge amounts, drive up the price, and resell at a profit. Or they might put a ton of orders in at high prices to confuse AH mods or any number of other practices. However, subscriptions are much more tightly controlled than normal AH interactions and none of the obvious tactics will work. Any time that is bought is locked to the account that bought it so no reselling is possible - the only people buying time are those that want to use it. Also players will not be able to list time at any price they want but rather the price will be set ahead of time and it will fluctuate based on demand. I don't know exactly how that will work but it means that players will have little control over the process aside from actually buying things they like.
This all means that people will have a great way to buy into the game and spend cash to get tons of resources without going to shady dealers. Gold sellers that focus on account stealing, hacks, and other unethical methods to get money will be starved out as their market is gutted by players going legit. Certainly some people will still buy stolen gold but I am pretty sure that market will be tiny by comparison. I don't think that there is actually much benefit to the game to having people who are rich in game be able to play for free but I don't think it is much of a loss either.
There are people all afluster about how this will mean all kinds of bad things for the in game economy but I don't buy it. They think that the possibility of making real money will cause people to suddenly start working extra hard to make gold but I can't see it - your ability to get real money out of the game is capped at $15.00 a month and that is a pretty pathetic return on time if you actually work hard to make that gold. Anyone who is collecting a fortune is going to continue to do so and people who didn't bother before aren't going to be suddenly interested when they can make real money at a fraction of minimum wage.
In the short term there will be huge market fluctuations. I expect a massive buy in from whales and I think there will be a huge spike in prices in the short term. However, once the initial purchase rush is complete the total gold in the economy will have dropped due to the AH cut and prices will rapidly settle back down to where they were before. I don't know how long it will take for that initial rush of gold transfer from the gold rich to the dollar rich to peter out but I bet it is a matter of days or a week at most. The smart money is in buying up a bunch of common mats (especially Savage Blood) and BOE epics that will be purchased with that flood of eager, flush new buyers. How *much* you can afford to stock is a question I can't answer though, and you had best be sure you aren't overpaying for epics in an effort to fleece the rubes.
Personally I am not going to bother speculating. In anticipation of price spikes I am going to lay in a store of two weeks worth of goods to guard against shortages but I am not going to try to play the Savage Blood or BOE market myself. I just want to make sure I can keep doing my thing, and that should be easy enough to do.