I tend to agree with Dave. Other than cleaning the barrel before shooting it, I haven't seen any difference in performance that I can attribute to a break-in procedure, and as many of you know, I am anal about measuring everything.

I have quite a few rifles that I bought new.
My first few new rifles I was religious about cleaning before shooting and performing a break-in procedure.

My last several new rifles, I cleaned before shooting and didn't do any break-in procedure - I just shot them.
The last few actually have performed better than the early ones that I "broke-in".
Admittingly, that is probably the result of already having a lot more trigger time with prior rifles before I bought the last few, or subsequent improvements in manufacturing tolerances at Savage.

All of them now have had thousands of rounds down their barrels and only my first new rifle has had the barrel replaced because of wear - over 6,500 rounds.

However, I have found that, for me, a new rifle will take getting used to before I can shoot it really accurately, whether I break-in the barrel or not.
The initial 100-200 round period with Savage barrels, that charlie b references in post #3, could be a combination of wearing-in the barrel and getting comfortable with the rifle's feel.
I doubt that anyone could determine what causes the initial improvement without shooting from a fixture, but I do believe that it takes some rounds to get into the groove with a new rifle, so I tend to think it is probably a bit of both.
For me, it also may be the result of finding what powder and bullet combinations that the barrel really likes to shoot.