According to a C&VG report games retailer GameStop will begin stocking Steam cards May 15 in a majority of its U.S. stores. The idea of having a retail card is that it gives those who do not have a credit card the ability to put cash towards a card they can use to buy games on Steam.
The report is based on this Kotaku report, which also includes a photo of a shrink-wrapped $50 Steam voucher card.
The only reason that this story is a bit weird is because GameStop bought Steam competitor Impulse from Stardock last year... While Kotaku's headline frames it as a "defeat" for GameStop, we see it as GameStop corporate saying that there's room for everyone to make some money.
Of course neither GameStop nor Valve has publicly confirmed the validity of the story.