Paul B. is Right
Aaron B.

I stand corrected. The Nazgul that stabs Frodo has a crown, and is subsequently referred to as the "dreadful King" by Aragorn. I think we can safely assume that this was the Witch-King, even without Gandalf's imprimatur.

This realization makes me even more incredulous that the Witch-King and his three Ringwraith henchmen didn't have the cajones to wrest the Ring from Frodo at that point.