There is a sci-fi book about a society of people who live in the
trees.  There is a fairy-tale that if you ever go to the ground, the
troll will get you, or something.  However, it is actually a
government consipiracy to keep people from discovering the secret
prisons on the ground.  I need to know the title and/or author.

Hello.

It sounds like you're describing "Below the Root" (1975) by Zilpha Keatley Snyder.

From a newsgroup message:

"... the "Below the Root" trilogy by Zilpha Keatley Snyder...  These
are enchanting books set in a world where everyone lives hundreds of
feet up in giant trees and moves about by gliding with specially made
cloaks(it's a low gravity world.)  Going all the way down to the
ground is forbidden except for a chosen few farmers and there are
rumors that strange beasts will capture and devour anyone who sets
foot on the ground..."
http://groups.google.com/group/alt.fan.harry-potter/msg/6d256d9c95d65e4a?hl=en&

Also see Amazon.com:
http://www.amazon.com/gp/product/0689304579/

Other books in the series:
"And All Between" (Green-sky Book 2) (1976)
http://www.amazon.com/gp/product/0689305141/
"Until the Celebration" (Green-sky Book 3) (1977)
http://www.amazon.com/gp/product/0812554809/

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search strategy:
google groups: "lives in trees" fantasy book

I hope "Below the Root" is the book that you had in mind.  If not,
please let me know via the "request clarification" feature.

I've just ordered a copy of the book but that definitely sounds like
it.  Thanks so much!