With respect to flying when a pilot certificate is required but the pilot has none, the Feds can seek fines and imprisonment. Such as this case: http://www.oig.dot.gov/library-item/5733
How many private pilots are sitting in jail for 91.17 violations? How about airline pilots who have been caught drunk in the cockpit? I have no idea, but I'm guessing the first number is quite low if not 0, the second I'd expect at least a few, but I won't bet on it not being 0.
"On November 4, 2011, Aaron J. Cope of Norfolk, Virginia, was sentenced in U.S. District Court, Denver, Colorado, for operating a common carrier under the influence of alcohol. Mr. Cope was sentenced to serve six months imprisonment, six months home detention and two years of supervised release. In lieu of a fine, he was ordered to complete 120 hours of community service and pay a $100 special assessment."