Some good, some bad
Jody, I wish I had more time this morning to completely answer your question - there is a lot to discuss on this complicated issue. I'll try to post some quick points throughout the day / week ahead.
The Oil Drum blog has some excellent articles and discussion threads about fracking, but they might be too detailed for a quick answer.
http://www.theoildrum.com/
But here something right off the bat...With rare exceptions, there really is no such thing as a completly impermeable rock unit. All strata is full of faults and cracks (called joints) that allow the passage of water and hydrocarbons. But I'll talk more about that later...