China Unicom, Apple’s exclusive carrier in the world’s largest cellphone market, announced Friday that it will begin selling iPhones equipped with Wi-Fi receivers on Monday.

This could be the breakthrough both companies have been waiting for. China Unicom, the country’s second-largest cellphone company with some 315 million subscribers, began selling iPhones in Oct. 2008, but government restrictions prevented it from marketing the high-speed Wi-Fi version sold in the rest of the world.

Those restrictions have since been lifted, and the 8GB iPhone 3GS that Apple released in the U.S. in June will become available to Chinese customers next week.

Neither Apple (AAPL) nor China Unicom (CHU) have issued sales figures for the iPhone in China, but news reports suggest that they have been overwhelmed by the country’s black and gray markets, which have been selling both imported and knock-off Wi-Fi iPhones for more than three years.

See also:

• Coming soon: 4-5 million Chinese iPhones
• China iPhone launch a ‘disappointment’
• Inside Beijing’s iPhone black market

[Follow Philip Elmer-DeWitt on Twitter @philiped]