BOSTON — Nisha Taylor was just about to put her beloved T-Mobile Sidekick in her bag. She thought the cell phone would be safer there than in her pocket. In the few seconds it took for the 18-year-old to unwind the string loop that held the Sidekick to her wrist, someone else eyed the device and made off with it.

"He just runs and he hits the phone," Taylor said. "The string pops. The phone goes up in the air. He catches it and he runs."

Although the Sidekicks — which have flashy flip screens and the youthful cachet of endorsements by rapper Snoop Dogg and basketball star Dwyane Wade — aren't among the country's best-selling phones, they might be the most stolen ones.

Boston police reported more than 300 stolen Sidekicks in 2008, accounting for 14 percent of all robberies in the city. New York City saw a 59 percent surge in subway robberies in December compared with the previous year, driven largely by thieves targeting high-end cell phones, especially the Sidekick.

And Adrian Portlock, whose company Checkmend.com tracks stolen cell phones, ranks the phone among the most-taken worldwide, even though the Sidekick's primary market is the United States, where it is available for $100 after a rebate.

Thieves have long targeted trendy items, from Air Jordans and Starter jackets to iPods and GPS units. But the Sidekick is not ubiquitous — it has never cracked the list of the five top-selling cell phones since the consumer research firm NPD Group began the ranking in 2005. Instead, thieves target Sidekicks because of their urban hipness quotient, and because they're easy to resell.

All T-Mobile phones use a Subscriber Identity Module, or SIM, card, a small computer chip usually stored behind the phone's battery that holds the owner's personal information, such as account data and phone contacts.

SIM cards are convenient for cell-phone users because to switch to a new device, they simply pop out the chips and put them into another phone that employs the same technology. All the consumer's saved information moves to the new phone.

But the reverse also is true: By removing a SIM card, a thief can quickly and cleanly erase the owner from the phone.

"It's more attractive to a thief if it's a SIM card-enabled cell phone," said Portlock, the chief executive of Recipero, whose Checkmend service charges customers to check whether electronic goods they want to buy or sell have serial numbers that are on a list of stolen or counterfeit goods.

Crafty thieves have learned how to manipulate the Sidekick so it can be used on other networks that also use SIM cards, such as the one run by AT&T Inc. Certain features — such as AOL Instant Messaging — may not work on the rival networks, but the limitations do little to discourage the switch.

And unlike in Europe, cell-phone companies don't share information when a phone is stolen, so rival companies may never know if one of their customers is using a stolen phone from another network, Portlock said.

Once a Sidekick is unlocked, thieves often scratch out the phone's identification number and sell the device on Web sites such as Craigslist or eBay.

"They have the maximum ability to turn them into cash," Portlock said. "It's that young market, Internet savvy, trend-driven market where they'll buy a used cell phone, no questions asked."

For its part, T-Mobile says it has "a long history of working with law enforcement agencies across the country on their investigations."

In Providence, R.I., where a majority of the roughly 190 cell phone thefts last year were of the Sidekick, Police Lt. Robert Lepre said the Sidekick's physical design makes users vulnerable. Most people hold the phone with two hands out in front of their body, using their thumbs to text or instant message on the full keyboard.

"When the kid's sitting there texting, it's pretty obvious what he's doing," Lepre said. "We've had kids that have definitely been followed and had their Sidekicks stolen."

The problem has become so pronounced that the Boston Police Department teamed with students from the Boston Arts Academy to create a poster — the winning one is emblazoned with the words "Hold on to Your Kick" — reminding young people to keep their phones close and avoid openly texting.

For Tatiana Mesa, the Sidekick's popularity — among her classmates and thieves — eventually turned her off. The 17-year-old student at the Boston Arts Academy recently switched to a BlackBerry Curve — after her Sidekick was taken while she was in class.

"I was like, I need something new," she said. "Let's see if it gets stolen."