I guess there are a few "break in" intervals that should be noted.
1 - Technically, the motors are run at the factory for an initial start up and probably the most important break in period. When the engine is new it can make a lot of internal heat (localized heating) as the parts are brand new and have not "polished" themselves together. This period is very important for the longevity of the engine and is therefore performed before the vehicle is sold. Duration = ???
2 - The break in period as described in the owners manual. A period in which the engine is supposed to be heat-cycled by normal used. This means to avoid long cruises at the same RPM as well as avoiding heavy loading of the engine. Again to limit the internal heating. At this time the cylinders still have honing marks and the piston rings are being seated as the wear. Duration ~ 500 miles (Whatever the manual says.)
3 - At some point in time (Around 10k from my experience) the cylinders and piston rings are finally polished together so that all the original honing marks are gone. The same would be true for other bearing surfaces in the engine as well as the complete drivetrain - trans, dif', etc.... This represents the full potential of the vehicle.
IMHO :D