Tom, Venus' phase IS visible to the naked eye
I have 40/20 vision and I can clearly see the phases of Venus* - at least that it is distinctly elongate in a "north-south" direction, and looks like a blurry mini-moon. Of course, it helps that I KNOW what it should look like)
Some people have markedly better eyesight than others, and Galileo's original telescope was very little better than the naked eye.
But dang, now I'm almost 50 and I'm losing the edge of my vision. Still, I had a good run for my money...