As readers of Technologyreview.com may know, I’ve been enthusiastic about the prospect of cell phones eating into iPod’s share of the digital music market. In fact, I’ve even come up with a rule (humbly titled “Hellweg’s Law”) that showed why this displacement would take place: The device that induces the least amount of pocket clutter wins.

Or, simply put, why would people carry two devices in their pockets – an iPod and a cell phone – if one could perform both functions well?

I still stand by that claim; but the device I thought could be first proof of this argument, Motorola’s ROKR phone, has let me down.

And I’m not the only one.

It seems that many people who purchased ROKR phones – which come with a mobile version of Apple’s groundbreaking iTunes Music Store – feel dissatisfied. According to research by Albert Lin, an analyst with American Technology Research, people are returning their ROKR phones at a rate “six times” the average for returns. Lin issued his report based on interviews with retailers, distributors, and call-center employees.

Nonetheless, Monica Rohleder, a spokeswoman with Motorola, stands by the product’s launch: “The Motorola ROKR is doing well in the marketplace – in fact, it is performing as well or better than the norm for other new mobiles from Motorola,” she told me in an email. According to Motorola statements, 250,000 ROKR phones were sold in the three-week period between the device’s launch on September 7 and the end of the third fiscal quarter on September 30. That translates into roughly 83,000 per week. During the same period, though, the wildly popular RAZR (part of the same product line as the ROKR, without the music capabilities) was flying off shelves at 500,000 units per week.

Even Motorola CEO Ed Zander has acknowledged some difficulties with the ROKR launch, specifically, with its marketing efforts. “We got off to a little bit of a rough start,” he said after last quarter’s earnings announcement. “People were looking for an iPod and that’s not what it is. We may have missed the marketing message there.”

The ROKR phone’s problems run deeper than a marketing foul-up, though. Because Apple didn’t want to cannibalize sales of its highly profitable iPod line, its agreement with Motorola limited the capacity of the ROKR to a mere 100 songs, compared with the typical iPod’s ability to store a thousand songs. Even if a user swaps out the ROKR phone’s memory storage card for a beefier one, the device still won’t hold more than this amount. That’s a pretty significant handicap.

So the deal with Apple turned out to be a Faustian bargain for Motorola. The company grafted Apple’s cool cachet onto the launch; but accommodating Apple’s business model has turned off a lot of potential consumers.

(When I contacted Apple for a comment on the iTunes component of the ROKR, the Apple spokeswoman sent me back to Motorola, saying: “That phone is their deal.”)

Despite the less-than-glowing response to the ROKR phone, I’m still bullish on the idea of a music-playing cell phone. For the concept to work, however, phone makers will need to put consumer interests first, notably, the ability to store lots of music on the device.

Actually, Motorola realizes this, and plans to launch a much more consumer-friendly cell-phone service, called iRadio, on January 6, 2006. The size of  the iRadio’s music library will be limited only by the amount of storage built into the phone. In addition, consumers will be able to download prerecorded “radio” programs (such as hits from the ’80s) to their phones, as well as sync the devices to car stereos for playback.

Although I’m not confident that consumers will go for the as-yet-untested radio channels feature, the new phone’s large music library storage capabilities should go a long way toward resuscitating the still-unproven concept of a music phone.