Funnily enough...
I heard an interesting theory about this which actually makes sence.
When running in an engine, technically, you're bedding in the piston rings to the cylinder so that they seal better.
Bearings rarely require bedding in because they run on heat treated hardened surfaces.
Now, the theory goes, when an engine is running, its the pressure produced in the cylinder upon compression/combustion that gets in under the piston ring, forcing the ring to seal on the cylinder wall NOT the spring tension in the ring.
So therefore, the first few hundred miles of a new engine is the most critical cause thats the 'bedding in' period and so for the rings to bed in well, you require LOTS of pressure, more than whats provided under normal riding conditions.
So by this theory, in the first few hundred miles, you should ride it like you stole it (within reason of course!) as the thrashing of the engine (and I use the term lightly) FORCES the piston rings to bed in better and therefore seal better.
http://www.mototuneusa.com/break_in_secrets.htm
Makes for an interesting read.