The chart at right from Horace Dediu’s Asymco blog shows the U.S. share of iPhone activations growing last quarter due, he speculates, to the U.S. launch of the iPhone 4S and the addition of a third domestic carrier, Sprint (S).

But the long-term trend is clear. The U.S. is becoming a progressively less important market for sales of Apple’s (AAPL) most valuable product line.

The company is likely to report a sharp uptick in international sales this quarter, reflecting the crushing demand for the iPhone 4S exhibited when the device went on sale in China last month. The spread will only get wider if China Mobile (CHL), the world’s largest mobile carrier, begins selling Apple’s next iPhone later this year.

Below: Another view of the same data, showing activations in units rather than market shares.