FORTUNE — If you thought the long green line at the top of the attached chart represented the burst of iPhone sales last quarter following the release of Apple’s (AAPL) new models, you could be forgiven. But you would be wrong.

When the NPD Group surveyed 5,000 American smartphone owners 18 and older last quarter, it didn’t ask them what phone they just bought or were planning to purchase. It asked them what phone they owned.

And 42% said they owned an iPhone.

The burst of sales last quarter probably helped push that number up from 35% a year ago. Samsung ownership also grew, from 22% to 26%. Google’s (GOOG) Motorola, Blackberry (BBRY) and HTC all saw their U.S. installed base shrink.

Still, the fact that iPhone penetration among U.S. smartphone owners has reached 42% is pretty striking, especially given Apple’s diminishing share of the world smartphone market.

There are two ways of looking at that fact:.

• Either Americans are carrying iPhones because — and only because — they are rich enough to afford a premium smartphone.
• Or (and this is what Tim Cook believes) everybody — even in very poor countries, like India — wants a premium smartphone. They just haven’t figured out how to pay for it yet.

CORRECTION: As several readers have pointed out, NPD’s claim is not that that 42% of adult Americans have iPhones, but that 42% of adult American smartphone users have iPhones. An early version had it as “better than four in 10 adult Americans own iPhones” when the correct percentage is 25% (0.42 x 0.60 = 0.252)