YMMV but I got an email under the Promotions tab in Gmail.
"EXPLORE GOOGLE PLAY $1 FOR YOU, ON US Enjoy $1 toward your next purchase on Google Play! From apps and games, to the latest music and movies, you're sure to find something you'll love. Click or tap the button below to add the credit to your account.*"
Most likely only usable for the Google Play account linked to the address that received the email.
This should probably go in the Wiki, but I don't know how to start one: My code was 59EGTVGYEQX1VTT