If you haven't heard by now, Apple's (Nasdaq: AAPL) iPhone 4 debut is officially the most successful product launch in the company's history, with 1.7 million units snapped up in the first three days. That's great for Apple, but let's talk about a very interesting battle playing out between AT&T (NYSE: T) and other carriers.

Some interesting research from Oppenheimer analyst Yair Reiner shows that AT&T is drawing fewer and fewer new subscribers with each new iPhone launch. When the revolutionary device debuted in 2007, roughly 18% of the buyers listed Verizon (NYSE: VZ) as their current carrier, 14% T-Mobile, and 18% Sprint (NYSE: S). Those percentages have generally fallen with each launch, and last week saw fewer than 10% of iPhone 4 buyers coming from each carrier -- and a whopping 80% from AT&T itself.

One explanation I've heard for the dwindling defections is that phones based on Google's (Nasdaq: GOOG) Android operating system are taking a firm hold among smartphone devotees, and the incentive to switch carriers to nab an iPhone is fading.

I see another reason, however. It's logical that fewer folks will switch with each launch until we eventually get down to those who simply won't go with AT&T no matter what. Some may live where it's just not an option. Others may prefer to avoid the dropped calls, weak signals, and other problems caused by the huge amount of data traffic being generated by smartphones and iPads. Some may hate the AT&T commercials with Luke Wilson. Whatever the reason, we're running into the law of diminishing returns here.

The big winner: Apple
Apple has played the AT&T relationship smartly, reaping great benefits from going with an exclusive carrier to this point. And now, it seems all but certain that Verizon and others will eventually carry an iPhone -- possibly as soon as the first quarter of next year, according to a Barclays Capital analyst quoted in Barron's. This will result in a flood of new sales and high fives in Cupertino.

So how long will the iPhone magic last? With AT&T starting to sell the iPhone in its stores tomorrow, and Best Buy (NYSE: BBY) and Wal-Mart (NYSE: WMT) still awaiting shipments of any significant quantity, we've got the near term covered. Then, look for another frenzy later this year if Apple ends its exclusive relationship with AT&T. Then, the launch with a new carrier will shatter more sales records. Finally, we'll get word of the next new launch, to happen sometime around this time next year.

Yep, you can bet the Apple momentum will carry well into 2011. It's good to be da King.

Fool analyst Rex Moore remembers when phones were attached to the wall with a cord-thingie. Best Buy, Sprint Nextel, and Wal-Mart Stores are Motley Fool Inside Value recommendations. Google is a Motley Fool Rule Breakers pick. Apple and Best Buy are Motley Fool Stock Advisor choices. Motley Fool Options has recommended a bull call spread position on Best Buy. The Fool owns shares of Best Buy. The Motley Fool has a disclosure policy.