According to a friend of mine who's in the know about such things, Tiger is using a specially designed TrueTemper DG X-100 shaft in his Ignite driver that is tipped between 2-3 inches, putting it in the XX to XXX flex range. (!)

Additionally, the average X-100 has 14 steps on the surface of the shaft. Tiger's has an average of 20 steps. (I verified this myself. You can see his driver on the cover of the new Golf magazine 45th anniversary edition. This shaft has 21 steps!) Each step position is painstakingly placed by TT according to the flex characteristics of his swing pattern and ball flight for the driver head that he's using.

Talk about a tune-up.