The details of how, exactly, this will play out still are to be revealed. I don't know, for instance, how its going to handle untagged mp3s. Or even music that was ripped from CDs - although I think its likely that most of these will be recognized the same way that iTunes uses the Gracenotes database when you rip a CD to iTunes.
With regard to the technology used in Shazam - tagging doesn't matter. The music itself identifies what it is.