Blizzard has already reduced the vendor price for cut green gems. While they left the uncut gems at 5g, it became much more profitable to shuffle all possible colors of gems into enchanting mats and blue jewlery or wow gold. The latest testing from the patch 4.2 PTR has reduced the price of uncut gems, too. On the live realms, they vendor for 100g a stack, leaving a decent vendor floor price for ore. Once this change goes live, they will be worth 50s each or 10g a stack. This should have the desired effect of causing fewer of these to end up vendored for inflationary vendor money with the potentially undesirable effect of making mining less profitable.
Bottom line is that the new "floor" price of Elementium Ore and Obsidium Ore is basically gone now. There are no more guaranteed sales, and if you flood a market by processing thousands of stacks of ore, you can't count on any vendor based fallback to at least get you your wow gold back.