Wal-Mart's Web site now lists the 16GB version at $97 with a two-year AT&T contract—down from $199 on Monday. Other outlets, including Best Buy and Apple's online store, continued to list the device at $199 as of early Tuesday.
Wal-Mart did not drop the price on the larger, 32GB version of the iPhone 3GS, which remains at $297.
The price cut is fueling speculation that Apple will launch the iPhone 4G at its WorldWide Developers Conference in San Francisco next month. Electronics manufacturers frequently cut prices on old inventory before introducing a new product.
The iPhone 4G rumors kicked into higher gear Monday when Apple confirmed that CEO Steve Jobs will deliver the keynote address at the conference. Jobs missed last year's event after reportedly undergoing a liver transplant.
Whether at WWDC or at a later event, the betting is that Apple will indeed launch a faster, 4G version of the iPhone, running iPhone OS 4.0, at some point in 2010.
Just weeks after an Apple engineer left one on a barstool in California, another iPhone 4G prototype was spotted earlier this month.
Bloggers at the Vietnamese tech site Taoviet posted pictures of a device, labeled APL0398, that was similar to the phone lost in March by engineer Gray Powell.
Taoviet's photos showed a complete teardown of the alleged iPhone 4G, including shots of its internal, system-on-a-chip. Apple, for its part, has not confirmed the pictures' authenticity.
Apple also isn't providing specifics about Jobs' WWDC keynote.
The company said only that the conference will feature five technology tracks, including Applications Frameworks, Internet & Web Graphics & Media, Developer Tools, and Core OS. Apple said the conference is now sold out, with about 5,000 developers expected to attend.