Essence of Airs have been a big gold maker for me.... most people have heard about Silithus being a great farmings spot and it is very much true. However I suppose a lot of people are put off by the fact that the Essence of Airs don't exactly fly off the shelf. People will tell you that it's a busted farming spot. Don't worry. Essence of Airs are a gold maker now, and Essence of Airs will be a gold maker come Cataclysm.
You see the Essence's are used to create the Icy Chill enchant as well as both 305 Agility enchants and come Cat I feel a lot of people are going to be looking for these enchants to place on Leveling/boa/twink gear so your sales should increase.
Right now the best spot in the game for these is Silithus, you should be able to farm 30-50 Essence of Airs an hour. I've been paying people to farm these for me for 9g per so even the farmers are making a decent bank. Currently I am selling these for around 20g each and selling 10 or so a day (some days 20+ some days just 1-5). Their deposit is minimal, and if you decide to farm these yourself a couple hours should net you about a weeks worth of supply. these are best farmed in the Northwestern corner as shown.