This could be extremely common knowledge or have been posted (I made sure to try four different search strings before I posted) but the plan I have for the expansion and gold is runecloth. Everyone is going to want a shot at the new Worgen or Goblin mounts ASAP.
If the vendors do go into the game as the Draenei and Blood Elf ones did in Burning Crusade the worst case scenario is 560 stacks of runecloth from neutral to exalted. With the inflated amount of gold some players have and the time required to amass that much runecloth (even through the Auction House) I believe there will be quite a few players who are willing to pay an absurd amount of gold for that mount.
Even if the vendors do not get put in you will still have an audience of brand new Worgen and Goblin characters that could be interested (as nothing has shown from the Cataclysm leaks that these turn-ins are removed from the original races).
Some tips:
1) Create a guild bank for storage. Not only does this save you space but it also can transition in to an easy sale (sell the guild bank - not 500+ stacks of runecloth).
2) This is very easily botted (see below for ideas of mobs).
3) If you have an enchanter this can also be a good money maker as the enchanting mats from the uncommons that drop from these sames mobs have gotten fairly expensive on most servers.
Mob list from WoWHead:
Runecloth - Item - World of Warcraft
Tyr's Hand and Sorrow Hill in the Plaguelands are two of my favorite spots as they are almost never occupied and have tightly packed, easy to AoE mobs. Sorrow Hill has a much more limited supply but I enjoy the break from grinding to tab out and read forums.
Overall Stratholme is a real solid bet too and much better for making other gold. Mobs do have a few annoying abilities like silences in there however.