I emailed the USCIS office yesterday and very promptly received a very polite response that our I-600A was approved last week!! Yay! My guess is that either it got lost in the mail or they mailed it to the old address. I am to contact them again if I don't receive it by the end of the week and they will mail another one. So by the end of next week our home study should be able to go to Korea. Woohoo!