It would drop the number of major American mobile carriers to three: Verizon, Sprint and AT&T.  Engadget reports:

"The combined customer base of this upcoming behemoth will be 130 million humans, though the agreed deal will have to pass the usual regulatory and closing hurdles before becoming complete."

Verizon and AT&T alone would service 70 percent of the American market.  ZDNet says the merger should come as no shock, as the two companies have been working closely for years:

"Technically and as far as infrastructure goes, the two should do well together. After all, they already are. If the deal goes through Sprint should be very worried." 

Om Malik worries about the control over pricing the new company would acquire:

"The Federal Communications Commission must have given an informal blessing to this deal.  There is a better than good chance that AT&T will make some nominal concessions in order to get the final approval from the FCC and the Department of Justice. In fact, AT&T executives have been rumored to be boasting about the nod-and-wink arrangement with FCC. It’s a shame!"

The NYT says that the debut of the iPhone pushed T-Mobile parent Deutsche Telekom out of the American market, shaping its decision to sell:

"Until Apple introduced its highly popular touch-screen device in 2007, which went on to become the world’s leading smartphone, Deutsche Telekom had been generating decent sales from its U.S. operation, with growth in some years surpassing that achieved in Germany.

But after the iPhone went on sale, sold exclusively at first by AT&T in the United States, T-Mobile U.S.A. began to lose its most lucrative customers, those on fixed monthly plans, who defected to its larger U.S. rivals — AT&T and Verizon Wireless, which began selling the iPhone in February."