Folks have started asking this so I thought I'd go ahead and post this up. I'm conversing with the Grand Master Leatherworker and in the training window the requirements for Master Leatherworking are:
I can only assume the other primary tradeskills are the same.
For the secondary skills, the book you have to purchase says:
Requires Cooking 300
No level requirement mentioned...
... So there you have it direct from the trainers themselves.