Apple's exclusive iPhone deal with AT&T won't last forever, according to Citigroup analysts, which means U.S. consumers might be able to choose from several carriers when they buy their combination iPod and smartphone.

While the exclusive agreement between Apple and AT&T has been successful so far, Apple will likely see less growth on the iPhone front since about 12 percent of AT&T's post-paid customers already own the handheld. Projections show that AT&T's post-paid iPhone customers will cap out between 20 and 25 percent, but other U.S. cell carriers have potentially untapped user bases ready to buy the iPhone.

Citigroup estimated that at a 20 percent penetration in the post-paid cell subscription market, Apple could pick up another 30 million iPhone owners by letting Verizon, T-Mobile and Sprint support the handheld. Verizon could sign on between 10 and 20 million iPhone users by itself over the next five years, according to Barron's.

Teaming with Verizon would mean the GSM-only status of the iPhone would have to go, and Apple would have to to produce a CDMA version, too. Such a move could increase Apple's costs since it would have to build two version of the iPhone, but it could also put the company in a better position to steal more market share from smartphone competitors such as RIM, Palm and Motorola that already offer GMS and CDMA-based models.

If Apple does strike deals with other cell carriers, it will likely impose requirements like it did with AT&T. Namely, carriers would be prohibited from co-branding the iPhone, they wouldn't be able to add their own start screen, users would get unlimited data plans, and revenue sharing on content downloads would be off the table.

Rumors have already surfaced that Apple and Verizon are negotiating an iPhone deal, but neither company is confirming that talks are underway. AT&T, however, has openly said that it is working to extend its exclusive iPhone deal beyond 2010.

With multiple cell carriers clamoring for the handheld, Apple stands to be the big winner, and customers may soon have more choices when they buy an iPhone.