Modern engines have special hone patterns and tolerances that decrease the need for break ins.

The best explanation that I have read goes something like this -

Engine speed should be varied every several minutes - acceleration causes extra fuel to wash cylinder walls down, while deceleration sucks oil and vapors up to lubricate cylinder walls. Repeats of this process wears the rings and walls to make a good seal.

Taking it easy for several hundred miles is good advice as well.

In Europe, in the '60s, new cars had stickers in their rear windows that read 'En roadage' that explained why the car's owner wasn't driving it like they stole it!