The rumours that have been circulating appear to be true: GameStop are now selling Steam vouchers in $20 and $50 denominations. The vouchers come with codes that you can redeem to add the funds to your Steam wallet. Handy for anyone who doesn't have a credit card, or perhaps who wants to limit their spending on the Summer sales. No news on availability outside of the United States.
Banner from the GameStop website
Full story including press release from Gamestop over at Joystiq.