Her name is Ellen Grossman....check out what she told the Washington Post:
Of not recognizing him right away, she said by phone that, “In general I have poor face recognition unless I know someone really well. I was aware Jay-Z was famous and existed, but I didn’t know how famous.” Since her celebrity sighting, she’s been doing her recon: checking him out on Wikipedia, looking at his Life + Times Web site.
“I’ve become much more aware of what he’s done and who he is,” said Grossman. “And as I checked him out, I realized, how embarrassing that I didn’t know who he was! There were all these photographers and a crowd of happy people around him, and security people. So it sort of dawned on me as I was looking around that he was someone famous.”
But how cool was it that Jay said, "Not very famous, you don't know me...but I'll get there someday"