Over the last 6 months, we’ve been hearing a lot of speculation that Sprint will be the next carrier to offer Apple’s popular smartphone. The gossip really kicked up in January when Verizon and Apple announced a partnership.

Sprint’s 3G CDMA network is very similar to Verizon’s offering, so an iPhone’s appearance on the nation’s 3rd largest carrier seemed imminent. Since then, we’ve heard all kinds of rumors suggesting that the iPhone was Sprint-bound. The latest one even pegs it to launch sometime before Christmas…

There have been several reports that Apple is making its next iPhone world-capable, meaning it would support both GSM and CDMA technologies. Combined with the leaked iPhone photos published a few months ago sporting T-Mobile branding, and I’d say a Sprint iPhone is more than plausible.

Citadel Securities analyst Shing Yin seems to think so too. Barron’s reported on a note from the analyst this morning, which predicts that we will see an iPhone on Sprint-Nextel before the end of the year. And why wouldn’t we?

With no known exclusivity contracts left on the device in the U.S,  why wouldn’t the Cupertino company bring its juggernaut to the remaining 2 U.S. carriers? There’s already over a million unlocked iPhones running on T-Mobile’s network, and it’s no secret that Sprint customers want in.

The interesting dynamic here is that both Sprint and T-Mobile still offer unlimited data plans. AT&T ditched the all-you-can-eat data option months ago, and Verizon is expected to follow suit this week. Will either carrier continue to offer unlimited plans once they see how data-hungry iOS users can be?

As always, analysts’ comments should be taken with a grain of salt. But it’s certainly not unlikely that we’ll see the next iPhone wind up on all 4 major U.S. carriers this year. The big question I have is, what about prepaid?

What do you think?

[MacRumors]