Goldman Sachs analyst Matthew J. Fassler told investors in a research note that Nintendo's latest portable gaming device (described here by Steven Mallas), which allows users to download games electronically, poses a "tangible early threat" to the physical sale of video game CDs and cartridges. He wrote that "While content will be limited at first, we believe it will likely ramp very quickly."

The Associated Press headline on the story was "Analyst: Best Buy video-game sales vulnerable," but I would say GameStop (NYSE: GME) is in much more trouble because selling video games is essentially GameStop's only business. While the decline of CD sales hasn't ruined Barnes & Noble (NYSE: BKS), it has absolutely murdered Trans World Entertainment (NASDAQ: TWMC), the operators of mall-based music stores like f.y.e.

It's too early to say when video game downloads will wreak havoc on brick and mortar video game stores, but I don't think there are too many people who would say that that will never happen.

Even with the stock near its 52-week low at 16 times earnings, that's a risk that long-term investors will want to pay close attention to.