For the records, I bought mine on Oct 13 and I got my $25 dollar credit, bought about 2 apps, and I just went and checked before posting and I still got $19 credit on my account.
Did you purchase from the Play store or from a retailer? I got mine from a retailer near the end of September, so it arrived after the $25 offer expired. My Play account still shows $0 when I go to the Redeem menu item.