One of the most interesting things happening in Cataclysm/4.0 is a change to Inscription. Glyphs are now permanent one-time purchases, but changing glyphs requires a [Vanishing Powder] (for 80 or lower) or a [Dust of Disappearance] (for 81+ characters).
What is really interesting is that both items can be crafted by scribes, but they are also sold by NPC vendors. Vanishing Powder is extremely cheap from the NPC, but it looks like a single Dust of Disappearance will cost 10g or so.
To me, it looks like Blizzard is experimenting with setting an upper bound on the player-to-player marketplace. If supply and demand lead to an auction house price of less than 10g, players will buy Dust of Disappearance from the marketplace. But if the price rises to above 10g, then people will switch to buying from the vendor.
This is really the first time that we have seen this behavior in WoW. Normally, NPCs serve as the lower bound for selling items on the AH. If the AH price of an item drops below the NPC price, you sell the item to the NPC instead.
I expect that Blizzard is doing this to keep the Auction House accessible to everyone. Buying and selling on the Auction House is fun, but if prices are too high, that makes the game less fun for the subset of the population with less gold. Theoretically, the upper bound serves as a brake, ensuring that no matter what happens to the market, necessary items are always available at a known maximum cost.
It's also possible that we may see a similar pattern for Enchanting Vellums. Currently there's only one type of vellum, and it is available from vendors for cheap. A second, higher-level vellum for an upper-bound price would mimic the same structure as glyph dusts.
Edit: I forgot one other aspect. On a low-population server, there are occasional shortages of items on the Auction House, just because there are fewer crafters, and the entire stock of an item gets bought out before a crafter can replenish it. When I played on Lethon, this would sometimes happen with flasks. It was never a permanent thing. Having the Dust being sold on an NPC guarantees that it is always available, that you are never completely blocked from changing glyphs.