Hello,
August of 2006 I went to the States for 3 weeks, and on my way back to my country (Spain) the flight company didn't ask for the i-94 form. So I ended up back in Spain with it.
I sent it to ACS - INS SBU, P.O. Box 7125 London, KY 40742-7125 USA (with the card, a letter, a passport...