The U.S. smartphone market is starting to look like a two-horse race, judging from data scheduled to be released Wednesday by a Nielsen general manager at AppNation III in San Francisco. Among the new findings:

• 44% of all U.S. mobile subscribers now own a smartphone
• 56% of the mobile phones purchased in the past three months were smartphones
• 71% or buyers purchased an “app friendly” — Nielsen’s phrase — Apple (AAPL) iPhone or Google (GOOG) Android phone
• 83% of the app downloaders in the past 30 days use either iPhones or Android phones

As Nielsen’s charts over the past nine months show, Android has grown to be the leading smartphone operating system (now with a 42.8% share), while Apple is still holding on as the leading smartphone manufacturer (with a 28.3% share).

The market shares of the alsorans — Research in Motion (RIMM), Microsoft (MSFT), Nokia (NOK) and Palm (HPQ) — continue to shrink.