Currently there is no cooldown on rare gem transmutes. Alchemist will be able to transmute 3 green gems and 3 herbs into one rare gem, also there is the Truegold transmute. I would say for the first few months, the best spec would be Transmute spec. If you have just one Truegold proc, that will worth thousands of gold alone. Rare gem market was really good when WotLK released do the lack of epic gems, and I think it's gonna be the same in Cataclysm, but now you will be able to mass produce those gems.