Editor's Note: This column first appeared in The Tech Edge , a proprietary newsletter, on Tuesday, June 24. For more information on The Tech Edge, click here .

Wall Street's fling with Nextel ( NXTL) will heat up this summer, but shrewd investors should brace for the moment when the winds change and the stock sails into rocky shoals.

By several accounts, Nextel is making solid progress on subscriber growth and is on track to post second-quarter results that will sate the bulls next month. Current expectations call for the company to report earnings of 24 cents a share on $2.5 billion in sales, a glowing endorsement of the company's industry-leading margins.

As testament to the Street's rapture, enthusiastic Nextel buyers have pushed the wireless phone service provider's stock up nearly 50% so far this year. That compares favorably to the 24% gain by Sprint PCS ( PCS) and the 38% increase of AT&T Wireless ( AWE).

But the second half of the year holds a couple of daunting challenges.

The most threatening development looks to be a direct attack on Nextel's niche, which, if successful, could not only slow subscriber growth, but rob it of customers outright.

The company has spent the better part of a decade improving its "Push-to-Talk" service, and recently announced it would offer the service nationwide. To date, there have been no significant challenges to the technology, supporting Nextel's claims that it is the only telco that can offer instant two-way radio communications. It has certainly been a good trick and the key to Nextel's success, but it also represents the company's technological Achilles' heel.

Starting as early as next month, Verizon ( VZ) will introduce new phones that imitate Nextel's beloved walkie-talkie features. Other rivals are also developing this service, which comes at a bad time for Nextel to lose its most differentiating feature, because starting Nov. 24, new number portability rules kick in that will allow customers to switch carriers yet keep their phone numbers.

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