Can stretch be improved? Most certainly unless there is a physical impediment. Think Chinese contortionists/acrobats, gymnasts and...classical guitarists. Almost all of their fleixbilty comes from training/practice, not inate body characteristics. And look at the reach of this guy's fret hand;
http://www.guitarlessons365.com/essential-hand-stretches-for-guitarists/ .
Check out Christopher's stretch tutorial,
http://www.guitartricks.com/lesson.php?input=19585 . There are other sources about stretching including the free Kindle book, Finger Exercises - Fingers, Hands, Forearms, Arms, Shoulders Therapy and Development Exercises Teaching Method,
http://www.amazon.com/Finger-Exercises-Shoulders-Development-ebook/dp/B00480ONWE/ref=sr_1_1?s=digital-text&ie=UTF8&qid=1358815551&sr=1-1&keywords=guitar+instruction (Kindle not required).
My stretch certainly has improved this year although I can't yet do six frets from fret 1. But I'm closer and I couldn't do five frets when I started. Primarily, I do this set of exercises from justinguitar.com (also free),
http://justinguitar.com/en/TE-007-FingerStretch.php after I do most of Christopher's exercises.
For most, playing and stretching exercises will improve one's reach. And with work on those I and most everyone else will get a bigger stretch and can voice what someone else posted in this forum, "I'll never be as bad as I am today."
"It takes a lot of devotion and work, or maybe I should say play, because if you love it, that's what it amounts to. I haven't found any shortcuts, and I've been looking for a long time."
-- Chet Atkins