Well, that just came right the flip out of nowhere. Google just sent out an email informing Google Wallet users that, as of September 17th, you will no longer be able to add funds to your Google Prepaid Card. After that, you have one month to spend any remaining balance, before it's no longer available. You can still receive a refund for the balance here, though, so Google's not just stealing your money. You will have to wait about 8 weeks to receive a refund, though. Also, refunds won't actually start until October 17th.
The search giant explains that this move is happening because you can now add any debit or credit card to Google Wallet.