Updated from 11:16 a.m. EDT

The exclusive Apple ( AAPL) iPhone deal has been a windfall for AT&T ( T). But that gravy train is looking a lot like a white-knuckle roller coaster ride as Apple turns up the pressure.

Adding to reports last week that Apple was exploring future device plans with Verizon ( VZ), Citi analysts wrote in a note Wednesday that Apple "will open its doors to multiple carriers in the U.S. as soon as late-2010."

As TheStreet reported last week, AT&T's exclusivity deal for the iPhone ends a year from now, in the second quarter of 2010. Verizon's leading chunk of the market is mostly a bargaining chip for Apple to extract even more out of AT&T, according to a source close to Verizon.

But Apple has pressures of its own. The pricey Macs didn't fare so well amid a recession. Apple's Mac sales growth stopped last quarter for the first time in five years, and the Mac's steady march toward marketshare gains actually reversed during that period.

Apple needs to tap growth elsewhere to offset its slumping computer sales segment. And while Apple and AT&T made iPhone magic for two years, Citi analysts point out, the rest of the U.S. wireless "market is too large to ignore."

This would be a potentially crushing development for AT&T's wireless juggernaut. Apple's iPhone is only one of AT&T's many devices, but Citi estimates that iPhone customers would generate 23% of AT&T's service revenue next year.

AT&T already pays Apple an estimated $350 in subsidies toward of the total cost of each iPhone. This initial outlay brings the price of the iPhone down to $199 and secures a two-year service contract worth $1,656 to AT&T.

AT&T has made the iPhone an anchor to its mobile data franchise with plans to link it with its U-Verse TV programming at some unspecific point down the road.

But AT&T may need to change that roadmap a bit.

"This issue is still 12-24 months away from impacting AT&T's operating results," Citi analysts wrote. "We believe the looming expiration of the exclusivity raises the urgency for AT&T to market successful smartphone alternatives to the iPhone."

Obviously, Ma Bell can always reach out and touch some smartphone partners like Research In Motion ( RIMM), Palm ( PALM) and Nokia ( NOK).

AT&T shares rose 0.7% to $26.69.

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