AT&T’s rivals have become more aggressive about their pricing, and that — paradoxically — could be good for Apple AAPL , according to Kaufman Bros.’ analyst Shaw Wu.

In a report to clients issued Monday, Wu notes that Sprint’s S Boost Mobile unit now offers a $50 plan that includes unlimited talk, messaging, Web, and walkie-talkie service. Deutsche Telekom’s DT T-Mobile, meanwhile, is test marketing a $50 unlimited voice plan plus $25 more for unlimited data/Internet.

So far, AT&T T and Verizon VZ are standing firm, charging a full $130 a month for unlimited voice and data.

That means that iPhone customers pay more than twice as much for all-they-can-mobile service as, say, T-Mobile customers using Google GOOG Android G1s.

How is that good for Apple’s iPhone, whose fate is tied for now to AT&T — at least in the U.S. market?

According to Wu, evidence shows that it’s the high cost of AT&T’s monthly service plans, not the cost of the devices themselves, that is the limiting factor keeping iPhone sales from growing faster than they are. (See here.)

He believes Sprint and T-Mobile’s price cuts will boost their lagging smartphone sales, which will in turn put pressure on AT&T and Verizon to cut their rates — to the ultimate benefit of iPhone owners.

“Overall,” he writes, “we view lower service plan prices as positive as it should help smart phone adoption maintain its healthy pace, even in this fragile economy.”

Wu is modeling Apple to sell 3 million iPhones in its second fiscal quarter, which ends in March, although recent checks with his supply-chain sources suggest that sales are tracking closer to 3.5 million.

He’s sticking with a price target of $120 a share, arguing that concern over possible management changes are mostly priced in and that a 15X price multiple these days is “reasonable and conservative.”

Apple closed Monday at $86.95, down 4.66% for the day.