In the last 50 or so times I have come in via air (primarily entry points are IAH, SEA, LAX, EWR, HNL, and DFW), 49 of those times I did not get stamped. I flew into Chicago this year though and he did not stamp me once but twice and as the ink was fresh, both stamps bled through (so it appears I have 4 stamps). He also stamped my customs form twice. It was 5 am and he had just come on shift so that may have something to do with it.

Nevertheless I really do not like my passport stamped as by the US as it is a waste of space to me. If i need proof I can show entry / exit stamps from other countries.