Correct me if I'm mistaken, but did not St. Peter himself deny Jesus not once, but three times?
He did. And the rest of his life, including the way he died, proved a complete repentance from those denials.
As a member of the Church of Jesus Christ of Latter-day Saints, I found a good article on that subject by Spencer W. Kimball, deceased president of the Church, called Peter, My Brother. Even if you don't believe Spencer W. Kimball was a prophet, as I do, you might find the article interesting anyway.
I never said such a denial was unforgivable, but certainly a big no-no. Though no man knows before the actual test what he will do, I certainly will not plan ahead to deny Him to save my life.