NEMA 5-15P blade holes
According to page 17 of ANSI/NEMA WD 6-2002, "The hole in flat blade is optional, and it is intended for manufacturing purposes only.  However if used it must be located as per dimensions shown above."

That doesn't really answer your question though.  Since they have specific dimensions, I'm guessing it's a hold over from a deprecated feature.  I'm about to replace some 60 year old outlets today, so I'll have to tear one apart and look for captive ball bearings, bumps on the wipers, etc.

Here is an example of a receptacle that locks using the blade holes.  http://fam-oud.nl/~plugsocket/NorthAm1L.html