Josh Lowensohn/CNET

Last week marked the one-year anniversary of Apple's iPhone 4 going on sale, and the countdown continues for whatever Apple plans to release as a follow-up. A history of four annual device releases would suggest Apple plans to roll out just a single new device, but one analyst suggests Apple's "one more thing" may, in fact, mean it will double up with two new iPhone models.

In a note to investors this morning, Deutsche Bank's Chris Whitmore suggests Apple is cooking up two versions of the iPhone for release later this year. One will be an honest to goodness next-generation device, he predicts, while the other would be a $349 souped-up iPhone 4 (which Fortune's Philip Elmer-DeWitt coins as a "4S") that would be available to customers who wanted to use it on prepaid cell phone plans.

"With Nokia and RIM struggling," Whitmore wrote, "the time is right for Apple to aggressively penetrate the midrange smartphone market (i.e. $300-$500 category) to dramatically expand its [total addressable market] and market share."

What's the big deal about prepaid, you might be wondering? Not everyone likes to get locked into a long-term agreement with their carrier versus paying money for a device up front. As my colleague Maggie Reardon noted in a story last week, there's also the fact that prepaid service providers such as Virgin Mobile, Boost Mobile, Cricket, and MetroPCS offer unlimited data plans that can cost as much as 50 percent less than what consumers might pay from "post paid" providers such as AT&T or Verizon Wireless. This also comes at a time when those big guys have been scaling back on "unlimited" plans.

Talk of a not-quite-iPhone-5-caliber update to the iPhone cropped up in May. Analysts, who were picking up hints from the supply chain, suggested Apple would be making modest refinements to the existing iPhone 4 model such as adding a speedier processor, improving the built-in camera, and slightly tweaking the exterior, much like the company did in the move from the iPhone 3G to the iPhone 3GS.

More recently, there have been reports that Apple is indeed planning to debut a properly overhauled device with the iPhone 5 moniker, and could even be working on a lower-cost version for developing countries, which matches up with Whitmore's assessment.

As for the timing on all this, numerous reports have suggested that Apple will offer an iPhone 4 follow-up to its customers in September, potentially bumping into (or being combined with) the company's annual iPod refresh.