Analyst for Kaufman Bros, Shaw Wu has written in a note to clients that Apple requires to bring another carrier onboard in the U.S. to continue iPhone growth but it doesn’t necessarily have to be Verizon.

Wu continues to explain that AT&T’s 90-million subscriber base is nearing iPhone saturation, Verizon has 93 million subscribers and would be a logical choice but would require a new iPhone model with CDMA capabilities.

Apple could also approach the other two main U.S. carriers, Sprint with 48 million subscribers and T-Mobile with 34 million.

Wu has been doing some research and looking into AT&T’s SEC 10-Q it mentions [PDF] that exclusivity with “a number of attractive handsets” could end. AT&T adds “we expect to continue to offer such handsets (based on historical industry practice), and we believe our service plan offerings will help to retain our customers by providing incentives not to move to a new carrier.”

For this reason Wu believes that Apple will begin entering into an agreement with another carrier in 2011 and “potentially Verizon in 2011 or 2012″. Shaw Wu adds, “We believe the argument for AAPL to pursue Verizon sooner than later is to address the growing presence of Android. What better way to do that than where Android has seen the majority of its success?”

With some purported inside knowledge Wu continues to say that, “Verizon negotiations are not finalized with important details still being ironed out”.