“We’ve been very, very focused on China,” Apple (AAPL) CEO Tim Cook told the audience at last week’s Goldman Sachs technology conference.

One result of that focus became clear on Monday, when China Telecom (CHA), China’s No. 3 mobile carrier and No. 1 Wi-Fi and fixed-line provider (total subscribers: 216 million), announced that it will begin taking iPhone 4S preorders next week for sales starting March 9.

How big a deal is this? To put it in perspective, AT&T (T) sold 7.4 million iPhones last quarter — Apple’s biggest ever — Verizon (VZ) sold 4.2 million and Sprint (S) 1.8 million.

As of January, China Telecom had 38.7 million 3G subscribers, roughly 15 million of whom, according to a recent Morgan Stanley report, are so-called high-end users who could easily afford — and are eager to buy — an iPhone. (See charts.)

China Unicom (CHU), the country’s No. 2 carrier, has more 3G subscribers (43 million), but roughly the same number of high-end subscribers as China Telecom. According to Morgan Stanley’s Katy Huberty, 20% of China Unicom’s users have already bought iPhones, a share she expects to eventually grow to more than 60%.

The big prize for Apple in China is still out of reach, however. China Mobile (CHL), the world’s largest carrier with more than 600 million subscribers, has been in discussions with Apple for years, and millions of its customers are already using jailbroken iPhones on its older 2G network. But sales won’t begin in earnest until Apple offers a 4G version of the iPhone that can run on China Mobile’s broadband network.

Huberty estimates that once that happens — perhaps as early as this fall — iPhone sales in China could quickly grow to 40 million a year.