T-Mobile today announced enhanced coverage in three major metro areas including Seattle, paving the way for the nation’s fourth-largest carrier to bring on Apple’s iPhone.

Customers in Seattle, Atlanta and Minneapolis will experience a “significant speed boost on our network” and will now be able to use their unlocked iPhones on the T-Mobile network. Bellevue-based T-Mobile expects to cover 100 million people with its new network by the end of 2012 and has improved coverage in 18 major U.S. metro areas.

Last week, T-Mobile’s parent company, Deutsche Telekom, announced that T-Mobile and Apple agreed to join forces and sell products next year. The nation’s fourth-largest carrier already has 1.5 million active unlocked iPhone’s in its network.

The company has been recently rolling out its HSPA+ 4G network improvements to enable faster speeds for customers that already have unlocked iPhones. T-Mobile does not offer 4G LTE, but the HSPA+ is comparable in terms of speed. Today’s announcement also notes that internal tests of unlocked iPhones running on T-Mobile’s 4G network recorded on average 70 percent faster download speeds than the iPhones on AT&T’s network.

The company will debut a 4G LTE network early next year, and it wouldn’t be surprising to see it happen at the same time T-Mobile launches Apple products.

UPDATE, Dec. 13 at 4:15 P.M.T-Mobile beefed up coverage in five more areas. The company has now enhanced coverage in 23 metros for over 100 million people.

Previously on GeekWire: T-Mobile, Apple will team up to sell products in 2013

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