Consumers won't find T-Mobile selling the iPhone 5 in its stores, but it's betting big on customers using unlocked iPhones on its network in Las Vegas.

The fourth-place carrier is throwing $4 billion into the pot to improve its 4G network and, as part of the plan, just switched on an HSPA+ 1900 MHz frequency that makes iPhone compatible in the Entertainment Capital of the World.

"As we retune our airwaves to turn on 4G service in our iPhone compatible spectrum in major markets this year, our network is becoming compatible with a broader range of unlocked devices," said T-Mobile CTO Neville Ray, highlighting the fact that Las Vegas is just the first city.

"With some sites live in other cities, including Seattle, Washington D.C., Los Angeles and the New York metro area, customers are already experiencing improved coverage and iPhone 'speed sightings' on T-Mobile's 4G network."

iPhone and T-Mobile 4G in the Marriage Capital of the World

In addition to being fully functional on the network, from the Las Vegas Convention Center in the north to the "Welcome to Fabulous Las Vegas" sign in the south, the iPhone on T-Mobile has some benefits over its officially compatible competition.

"Our internal tests of unlocked iPhone 4S devices running over 4G on our 1900Mhz spectrum recorded on average 70% faster download speeds than iPhone 4S devices on AT&T's network."

That advantage won't last, however. AT&T, Verizon and Sprint are broadening their LTE coverage, while T-Mobile's plans list LTE as "coming in 2013."

Of course, that's significantly faster than the 2G speeds T-Mobile customers have had to deal with on unlocked iPhones previously. And the compatibility should be made even easier with a T-Mobile nanoSIM supposed to arrive in October.

There's no telling when or if T-Mobile's efforts to improve its network will result in it officially selling the iPhone. But it's one step closer to having to stop selling against the iPhone.

Via AllThingsD, T-Mobile