Tobold, The WoW Economist, and Greedy Goblin have noted that there appears to be some deflation going on in Auction House prices at the moment. Each of them attributes it to a different cause. The WoW Economist says it's because of a temporary decrease in demand because most people are levelling. Tobold feels it's a permanent decrease in demand because of the lowered difficulty of WotLK PvE. I don't really understand Greedy Goblin's explanation, but I think he's blaming it on the rich people gouging the poor people (Marx-style class warfare?).
I'll offer a different explanation: Right now, the market in WoW is not obeying supply-and-demand in the classic sense, but is being dominated by the mechanics and side-effects of levelling professions.
Here are my assumptions:
1. Most people are gatherer/crafter. They have one crafting profession and the related gathering profession. There are some double gatherers or double crafters, but they are outweighed by the gatherer/crafters.
2. Most crafters have a personal stockpile of raw materials. They primarily craft using materials from that stockpile and only purchase materials if they are missing them.
What I think is happening is that most raw materials being gathered never enter the market. Instead they are stockpiled to fuel the crafting required to level the profession. Only after the crafter hits 450 skill, will excess raw materials be redirected to the market.
So on the raw materials side, what we've seen is an artificial scarcity of materials, as most materials were being reserved for levelling. Now, a month after the expansion launched, more and more people are hitting 450 skill, and excess materials are starting to be sold off for profit, decreasing the price.
On the crafted materials side, the fastest crafters were the only ones able create the higher level items, and thus could command a premium for their items. Now, more and more crafters are reaching the higher skill level and producing the high level items. The big change here is that the crafters don't care if they are selling the item for a loss. They are crafting for skill points, not monetary gain. Any money gained from selling the item is a bonus. Supply is independent of demand at this point. Even if all the tanks have Tempered Saronite, it will still be made, because it nets the blacksmith a skill point. So a serious excess of supply is being produced, again driving down the price.
The key here is that the crafters are not being driven by traditional monetary incentives, they are being driven by the desire to increase their skill. That warps the market. I think this state will probably last for the next month or two, until a great majority of the crafters have reached 450 skill. Then levelling skill ceases to be a concern, and the traditional economic concerns of profit and supply-and-demand will reassert themselves, and we will see prices stabilize.