But it may not be a done deal. Because of its size, it’s likely to get a close look from U.S. regulators, says .  “Or the deal could be blocked completely -- last year was the first year that the FCC did not conclude the U.S. wireless industry was ‘competitive,’” Frommer notes. “The companies expect the deal to close in ‘approximately 12 months’ -- a long time.”

The deal could trigger a realignment in the U.S. cellular market. There’s already speculation that Verizon might bid for Sprint Nextel, both using CDMA cellular technology, and regional carriers could band together to create a low-price, innovative counterweight.

AT&T’s cellular spectrum covers 850 and 1900 MHz for GSM/GRPS/EDGE and for its expanding UMTS/HSPA and increasingly HSPA+ 3G services. T-Mobile uses mainly the 1900 MHz band for its GSM-based services. In late 2008, the company began deploying its UMTS 3G service in the 1700/2100 MHz AWS band. It’s been aggressive in deploying HSPA+ technology, which, the company claims, can boost average download speeds to 21Mbps from 7.2Mbps and create more reliable connections.

Both companies are taking a two-pronged approach to cellular data: upgrading 3G services to support HSPA+, and moving toward LTE. that it will begin building out its LTE network in the summer of 2011,  with the initial nationwide phase completed in 2013. Late in 2010, AT&T paid Qualcomm nearly $2 billion for the latter’s 700 MHz spectrum holdings, enough to cover 300 million people nationwide for LTE service, . 

In an interview Sunday with Ina Fried of All Things Digital’s Mobilizer blog, AT&T President Ralph De La Vega cited the dovetailing of spectrum holdings as a key reason for the deal. “The first thing is this deal alleviates the impending spectrum exhaust challenges that both companies face,” De La Vega says. “By combining the spectrum holdings that we have, which are complementary, it really helps both companies.”