Is Snape really the Half-Blood Prince?

Quote	Source
"How old is this book, Harry?" "I dunno, I've never checked." "Well, perhaps that will give you some clue as to when the Prince was at Hogwarts," said Lupin. ... "...Harry delved into his trunk and pulled out his copy of Advanced Potion-Making before getting into bed. There he turned its pages, searching, until he finally found, at the front of the book, the date that it had been published. It was nearly fifty years old. Neither his father, nor his father's friends, had been at Hogwarts fifty years ago."	Book 6, Page 337 Half-Blood Prince, Chapter 16, "A Very Frosty Christmas"
This was Hogwarts as Riddle had known it, meaning that this unknown wizard was headmaster, not Dumbledore, and he, Harry, was little more than a phantom, completely invisible to the people of fifty years ago.	Book 2, Page 243 Chamber of Secrets, Chapter 13, "The Very Secret Diary"
"And this diary is fifty years old," said Hermione, tapping it excitedly. "So?" "Oh, Ron, wake up," snapped Hermione. "We know the person who opened the Chamber last time was expelled fifty years ago. We know T.M. Riddle got an award for special services to the school fifty years ago."	Book 2, Pages 232-233 Chamber of Secrets, Chapter 13, "The Very Secret Diary"
"You dare use my own spells against me, Potter? It was I who invented them - I, the Half-Blood Prince!"	Book 6, Page 604 Half-Blood Prince, Chapter 28, "Flight of the Prince" Spoken by Snape
PONDER THIS
..."tryng to make himself feared using the Dark Arts, gave himself an impressive new name - Lord Voldemort - the Half-Blood Prince - ..."	Book 6, Page 637 Half-Blood Prince, Chapter 30, "The White Tomb"