As Sprint, the nation’s third-largest carrier, begins its long-overdue nationwide 4G LTE rollout, those keeping their fingers crossed for some crazy fast download speeds on a next-generation 4G LTE iPhone will be in for an unpleasant surprise.

While the carrier will keep unlimited data, whether or not the next iPhone has LTE (and we know the Now Network will be eligible for a 4G LTE iPhone), the handset will theoretically achieve much greater downloads speeds on rival AT&T’s and Verizon’s networks.

Here’s why…

Earlier this morning, Sprint shared some updates during a breakfast at the CTIA Wireless trade show. We heard earlier they’ll flip the switch on 4G LTE in Atlanta, Baltimore, Dallas, Houston, Kansas City and San Antonio in mid-year 2012, with other markets to follow later this year.

But Sprint’s 4G LTE won’t be as fast as rival Verizon’s or AT&T’s because the rollout will initially be in 5-MHz channels versus Verizon’s 10-MHz channels.

Writing for PCMag, Sascha Segan lays it out nicely:

Peak speeds won’t hit the levels we’ve seen on Verizon and AT&T of 30-40Mbps. But the network will be more consistent and more convenient, with reliable average speeds and faster handoffs between 3G and 4G than Verizon is seeing.

In fact, Sprint is managing 4G LTE rollout in a way which won’t require a redesign of existing iPhones, the author notes.

Shifting voice calls to the new 1x-Advanced technology lets Sprint carry more calls in less spectrum, opening up airwaves for more 3G data on devices like Apple’s iPhone. Faster, Ethernet-based backhaul dramatically increases the number of megabits each cell site can carry.

Moving voice calls to abandoned Nextel 800-MHz spectrum opens up even more 1900-MHz data bandwidth for hungry iPhone users, without the iPhone having to include another radio band.

Also, this nice-to-have:

Sprint is also working on background apps that will automatically connect its phones to Wi-Fi networks, further improving the apparent data performance of 3G phones.

If it wasn’t for the iPhone, Sprint would have actually lost 400,000 customers in the first quarter of this year. They activated 1.5 million iPhones, 44 percent of which were to new customers, or approximately 660,000 new iPhone subscribers.

Despite its success with the iPhone, board of directors punished Sprint CEO Dan Hesse for betting the farm on Apple’s handset, forcing him to agree to a pay cut by a whopping $3.25 million in this year alone.

Last year, Sprint prepaid an astounding $15.5 billion to Apple for the iPhone, the move some watchers opined put the company at a risk of bankruptcy.

Sprint, like other carriers, is having issues with high iPhone subsidy.

Apple’s device carries some of the heftiest subsidies in the industry, about $200 higher than other manufacturers.

While this report will admittedly disappoint some of you, the fact remains that Sprint is the only major carrier in the United States to offer unlimited data to its 4G LTE customers.

That’s gotta be worth something, right?