The short, and obvious, answer to that question is “a hell of a lot.” In a note to clients today, Bernstein Research analyst Toni Sacconaghi argues that Wal-Mart (WMT) can sell between 800,000 and 1.3 million iPhones in 2009, though the retailer’s low price mantra may appear inconsistent with the iPhone’s market positioning. And that would seem to be a reasonable estimate. After all, Wal-Mart has some 2,500 retail outlets in the states, through which 100 million customers stream each week.

“The addition of Wal-Mart will roughly double the iPhone’s distribution in the US to nearly 6,000 outlets,” Sacconaghi writes. “We believe iPhone sales will benefit from the added distribution, though not proportionately; in our view, price (for both the device and service plan) is still the biggest hurdle to mass adoption…. With the iPhone already available at 3,000 Apple, AT&T (T), and Best Buy (BBY) stores, Wal-Mart likely does little to expand the device’s geographic reach. However, we believe Wal-Mart will have a greater impact on the iPhone’s demographic reach in terms of raising awareness and availability among lower-end consumers who are less likely to shop at the Apple Stores or BestBuy.”

One last point worth noting here, Sacconaghi puts little faith in rumors that Wal-Mart will offer a $99 4GB iPhone. “Could there be a cheaper iPhone at Wal-Mart,” he writes. “We think it unlikely, at least initially, but the idea is not completely unfounded. In our view, a $99 price point for the iPhone seems too low, but some modest discount is not implausible.”

First, Best Buy. Now Wal-Mart. Can Costco be far off?