Just to throw it out there, here's what I have gleened from my reading about impact dynamics from a clubmaking perspective. It was interesting that the "prestressed" condition is considered bent away from the target, because it is pretty well documented that for 95% or more of golfers the shaft at the head end is actually bent forward (and down) at impact. There's video, stills and deflection graphs (True Temper Shaft Lab) that have shown this.
Regarding the deceleration issue, C&S tested a clubhead on a hinge at the end of a shaft and concluded that it made no difference to the ball flight. The still impact pictures are very interesting. Basically, they conclude (as have others including Jorgensen mentioned by Steph) that the head at the point of impact basically acts as if it is disconnected from the shaft. That's why in the impact formula (conservation of momentum), the weight of the shaft is not considered - only the head. C&S also figured that the player doesn't feel the impact until the ball is well on its way.
I don't think this at all negates the imperatives of TGM - these factors in my view aren't really necessary to support the model and I'm guessing HK did not have access to some the equipment that has been used to characterize impact and shaft bending.
One area where the physics guys do basically support Homer is in the ball flight laws in which TGM is somewhat at odds with the more popular ball flight laws put forth by John Jacobs and others but that's probably another discussion....
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