Back when Google Wallet first launched, Google also launched their own, pre-paid credit card for use with the mobile application. Then, just recently the service was opened up to allow the addition of other credit cards. Today, we're receiving emails in our inbox from Google themselves with further Wallet related news. As a result of opening up the service to other cards, the Google pre-paid card is going away. So you need to spend that money, and spend it soon.
It isn't just over, just like that, though. There are some key dates provided that run down to the discontinuation of the card. On September 17, additional funds can no longer be added to the card. Then, following behind a month later on October 17, the card will disappear completely from Google Wallet. If you still have remaining funds on there though, don't panic, you will be able to request a refund.
One further point to note however, is that resetting the application after September 17 will also remove the card. And it can't be re-added. So, probably better to spend your money first.
