In what amounts to another major milestone for the smartphone industry, the latest report from Nielsen shows that smartphone purchases now comprise the majority of all new smartphone purchases in the US.

Nielsen’s May survey of US mobile consumers shows that 38% now own smartphones. Noting substantial double-digit growth in purchase rates, Nielsen finds that Apple’s iOS is leading the demand for smartphone purchases while Android (still most popular) is now “flat.” RIM, meanwhile, continues to lose ground.

Android continues to be the most popular smartphone operating system, with 38 percent of smartphone consumers owning Android devices. However, while Android also leads among those who recently purchased a new smartphone, it is the Apple iPhone that has shown the most growth in recent months.

Most interestingly, however, more than half (55%) of all the new handset purchases in the last three months in the US were smartphone purchases. Just twelve months ago, only 34% of new cell phone buys were smartphones during the same three-month period last year.

Ultimately, as the data reveals, feature phones may now have entered into the full-blown start of a precipitous decline from which the category of mobile phones won’t ever completely recover.

For more information on the study, be sure to check out the NielsenWire.