[ageitgey]

It's 100% illegal to present a non-US passport at the US border if you are US citizen.

The law is 8 U.S.C. 1185 - "it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport."

In the past, the penalty for violating this has generally just been "a stern talking to," like you said. But no guarantees on that.