With Mists of Pandaria on the horizon, a lot of crafting items are rapidly dropping in price on the auction house. Many of these items will lose a lot of their demand when the expansion comes out. Some of these items, however, will continue to be sought on the auction house by people who are leveling professions. If you have the bank space, and the gold to invest, you can make a nice profit stockpiling these items when you flip them on the auction house after Mists of Pandaria's release.
Hypnotic Dust is a necessary component for leveling Enchanting through the Cataclysm levels. Every enchanter is going to need several stacks of dust to get from 425 to 500 Enchanting. This is an incredibly easy item to flip as you don't have to do any crafting to profit with it. Buy it cheap, stockpile it until MoP, then sell at a huge profit!
Volatile Earth & Volatile Life
Volatile Earth is rapidly dropping in price as the demand for crafted items comes to a crawl, and as everyone prospects their stores of Pyrite Ore. As with previous expansions, there will always be a continued demand for elemental items while leveling professions. Volatile Life can be transmuted into other elements, and both Volatile Life and Earth are wonderful candidates for post-expansion AH flipping if you can get them cheap.
Cinderbloom & Whiptail
If you can spot cheap stacks of these easy-to-farm herbs on the AH, save them to flip later at a nice profit. Both Alchemy and Inscription demand large quantities of these for moving through the Cataclysm levels, and they will not be farmed as heavily after MoP's release. Having demand from two professions helps cut your risk significantly when investing in these.
Pets, Pets, and More Pets
Mists of Pandaria will bring the new Pet Battle feature – both increasing demand for pets, and making pets trade-able in the process. Having a large stockpile of pets ready to sell on the auction house will save you time when MoP comes out. Stock up on vendor pets, Engineering crafted pets, and any cheap rare pets you can find on the auction house.
Bolts of Netherweave
Okay, this one isn't a Cataclysm item – but it still shouldn't be ignored. A new expansion means lots of new toons being created. While it's also possible to carve a niche with 18, 20, and 22 slot bags, the 16 slot Netherweave Bags remain the most economical and a favorite for alts. Netherweave Bags and stacks of Netherweave Cloth can take up a lot of bank space, so the best way to stockpile Netherweave is by turning it into bolts and saving the bolts to craft into bags later.