Same thing happened to me when I was in transit in LA about a year ago. Had the green card still stapled into my passport and being the goody to shoes I am I never ripped it out. The nice lady at immigration took a sensible approach and said that really she shouldn't let me in as I could have technically overstayed since the card wasn't taken off me when I departed the last time I was there. Then she gave me a printed set of instructions complete with an address somewhere in the US that I should send proof that I left the US to. The form was quite helpful and gave examples of credit card statements, boarding passes etc. In other words anything to prove that you left the US and didn't overstay. No drama at all.
Now that reminds me. I never did get around to sending off the stuff....Doh.