SAN FRANCISCO (Business 2.0 Magazine) -- To succeed, you've got to be thin, smart and versatile.

That might sound like a maxim from the world of haute couture. But the same thinking is spreading to the cell-phone business.

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And it makes sense: We carry our cell phones practically every waking moment. Why wouldn't we care about their looks? The era of the handset as a purely utilitarian tool is definitely over, and the devices have become an expression of their owners' fashion sense and personality.

The fashion trend is one that handset manufacturers and mobile operators in a U.S. market nearing saturation are happily spurring along, by enlisting designers, engineers and even social scientists to create hip and fashionable phone after hip and fashionable phone.

For this fall, the cell-phone industry's fashion gods - in this case, industrial designers and mobile-market analysts - have decreed thin to be in. In fact, the ultrathin Motorola (Charts) Razr's success has spurred a word to describe the push to make phones slimmer and slimmer: "Thinism."

Thinism may prove to be a fad of the moment. But faddish, must-have-it-now devices mean that cell-phone users replace their phones more frequently. In developed markets such as the United States, spurring more frequent replacement purchases is where the money's at.

Though research firm Strategy Analytics estimates global handset shipments will reach a whopping one billion units in 2006, it's in emerging markets that most of the new customer acquisition action is happening. Here at home, it's our appetite for new, improved and stylish devices that drives sales.

Unlike couture, when it comes to cell phones, looks aren't everything. How phones feel when you use them matters quite a bit. But looks definitely count for something - why else the Razr come in multiple shades of pink? Here's a rundown of this fall's hottest cell phone trends:

Motorola's Razr may have spawned the slim-phone revolution back in 2004, but other handset makers are quickly catching up.

At a waist-cinching 13 millimeters, Samsung's recently introduced D900 is purportedly the thinnest slider of all time.

"We're focusing on slim phones that have substance," says Kim Titus, a Samsung spokesperson. The Korean company plans to launch eight phones in the next eight weeks, many of which Titus says will fall under the category of thin designs.

"What we've got here is a bunch of 'me-toos' playing catch-up with the Razr," says Howard Nuk, an associate creative director at Palo Alto-based Frog Design, a product-design and branding consultancy. "But a lot of these guys are doing a great job."

We'll come right out and say it: The much-hyped LG Chocolate music phone from Verizon Wireless (Charts) is all beauty and no brains. It looks a lot like an iPod, down to a circular navigation setup that resembles iPod's click wheel, but that similarity just confuses matters. The user interface makes the phone nearly impossible to use, especially if you try to use it like an iPod.

But the device is nevertheless a sleek-looking work of art. One hopes Verizon and LG can work out the Chocolate's interface. The phone is also part of a growing trend towards phones that look more like a digital camera, MP3 player, or handheld videogame console, rather than a generic cell phone.

"Before, we had a phone with a camera in it," says Nuk. "Nowadays, we're starting to have a very high-megapixel camera and oh, by the way, there happens to be a phone in it."

If you're not that pretty, you'd better be smart, say industry analysts. That means more smartphones.

"They're just getting smarter and more powerful, and will continue to grow," says Bill Hughes, a principal analyst with research firm In-Stat.

Palm (Charts) just announced the Treo 700wx, a Windows Mobile phone available on Sprint which has more memory than the Treo 700w, but otherwise looks the same.

The gadget blogs are buzzing about a rumored Treo 750. Palm remains tight-lipped, other than to say a next-generation phone will be out before the end of the year.

Other handset manufacturers are jumping on the smartphone bandwagon too: Nokia (Charts) is set to launch its much-hyped E62 later this fall, which is expected to put e-mail and other smartphone functions into a compact package.

With the likes of Dolce & Gabbana and Versace dabbling in mobile phone design, it's clear handsets have earned their reputation as fashion accessories.

Expect to see more designer-phonemaker collaborations this fall, in the form of limited-edition models.

Limited-edition or "fashion" phones represent a tiny portion of overall sales. Motorola reportedly made and sold just a few hundred of its recently launched gold-hued, Dolce & Gabbana-designed Razr.

But the small, get-'em-while-they're-still-in-stores quantities are part of the the allure of the limited edition phone. And the instant cool factor achieved by partnering with hip designers is can give your brand a halo.

It's not unlike the dynamic in the fashion world, where runway fashions set the tone for the mass-market, ready-to-wear pieces that show up in department stores. Even just a few high-end phones that land in the right celebrity hands can work wonders for sales.

And as fashionable as it gets, the business of making cell phones - even good-looking ones - is still all about the bottom line.

Click here for a gallery of the latest cell phones.