I went leatherworking/skinning for my rogue. I'm at 225 now, and I've realized that A) I'm never going to wear any of the crap I make cause I can buy better, and B) I barely make any money cause the auction house is flooded with the same things that every other leatherworker and their mother is making. I assume the same goes for blacksmithing and tailoring.
Engineering is of limitied utility for any class, but especially for a mage. Enchanting might be useful to make money off of, but its VERY time consuming. Alchemy is alright, but since it doesnt stack with effects, and you can certainly do fine without them, and buy the ones you need from others, is also very useless, and herbs dont sell for much.
My suggestion is to go skinning and mining all the way, and just sell what you farm, and buy the stuff you need. Been working great for my druid so far...I've never had so much money at so low a level before.