Yep. OoTP doesn't specifically say the locket was thrown out (which would be Kreacher), it only implied it (which would be Mundungus.)
I can't wait for Book 7! Started HBP again on the train this morning, reading VERY slowly, and with the assumption that every word is significant.
My vote is on Mundungus. Since Harry now owns Kreacher, he would be able to simply order Kreacher to return the locket and that would just be too easy.