In a long enough timeline, everything on the AH will be massively devalued, helping the "poor" players immensely. As more people level up and start farming, the market gets more flooded with items and with everyone being able to compete in an open market, prices tend to drop. In wow that situation was avoided by having items become "soulbound." So if you wanted mr uper badass helm of godliness, for 4 diff characters, you had to buy 4 diff ones and equipment was basically consumable. In D3, at least so far, items are not soulbound, so you could buy 1 helmet and just transfer it around character to character via the stash. This makes the demand a VERY fixed level of 1 per person (except for the rich who want to buy a 2nd due to laziness, not due to need). So eventually everyone will have everything, cheaply.
Oh another issue that has to be brought up. Even though Blizzard says they won't be listing things on the RMAH, really what's to stop them? They are setting up the perfect system where people are going to be willing to pay money to use a virtual item (exactly like selling pets and mounts in WoW) only now people are going to be willing to pay far more for these items as they will be useful in game. I think Blizzard would have to be INSANE to not set a fixed amount of items per month as additional revenue streams. They would have to be careful as to not impact the economy very much, but it wouldn't be hard, they have smart people working there.
I am will to put money on it that Blizz will eventually change it so items do become soul bound. Whether its before release day, or after. They will probably do it. Because the differences of the amount of money they would make between soulbound items and non-soulbound items is huge. Blizz would make a lot more money if they had soul bound items. I personally would amost like it more if they did do it because the AH will then be a little more stable and everything wont be dirt cheap.