From what I've heard, almost all cases of gas getting into drinking water tables is through poor cement jobs. Put it this way: the gas zones have had millions of years to equalize with zones around it. They're at a higher pressure than the formations above them and if they weren't sealed the gas would have escaped to the aquifers, and probably the surface, eons ago. Now a well gets drilled to them and gas is found to be finding its way into formations far above them. While it is possible that the frac job has made a fracture all the way to this, that far less likely than gas being able to find its way through voidage in the cement job in the well bore. I suppose that frac fluids could also make their way through the casing in this matter, but not as easily as gas.
The number of problem wells is relatively small, probably much less than 1%, but because there are so many of these wells being drilled dramatic incidents start adding up. Maybe in their rush, service companies are getting sloppier with their cementing jobs, but I haven't heard any evidence of this. I don't really know the best response to the problems. America needs energy, and gas is a very attractive way of generating it, far better than coal. Plus the shale gas industry brings money to regions that can really use it at the moment. But on the other hand, people's drinking water keeps exploding. Hopefully the problems can be brought under control and the massage resource that is shale gas can be exploited to its potential.