Maybe all it indicates is that un– was really a proclitic instead of a prefix all along, but we never knew it until someone came up with this usage that revealed it.    From Wordnik.com.  [The Volokh Conspiracy » The Trouble with Some Prescriptivist Claims:] Reference

Not what you would expect from a proclitic … Also, “I’m” would be a bit funny then.    From Wordnik.com.  [Whoever v. Whomever! Cases collide! Match of the Century! « Motivated Grammar] Reference