The results of a Nielsen survey conducted in August and released Monday reinforced several ongoing trends:

• The smartphone pie is getting bigger. While 43% of all mobile subscribers in the U.S. had a smartphone as of August, according to Nielsen, 58% of those who bought a new mobile phone in the last 3 months chose a smartphone over a feature phone.
• Android’s slice continues to grow. The share of smartphones running Google’s (GOOG) Android OS hit 43% in August, but of those who bought smartphones in the past 3 months, 56% bought Androids.
• The iPhone’s share is holding steady. Apple’s (AAPL) iOS remains in second place, with a 28% share of all subscribers and of recent purchasers.
• The also-rans are running out of time. Research in Motion’s (RIMM) BlackBerry and “other” continue to lose share.

In its interpretation of the results, Nielsen offers two contradictory observations:

• “The preferences of … so-called ‘recent acquirers’ are important as they are often a leading indicator of where the market is going.”
• “[Apple’s] figures could change quickly in the months to come. Every time Apple launches a new iPhone or makes it available on a new wireless carrier, there is an increase in their sales.”

Below: A Nielsen chart showing the relationship between all subscribers and recent acquirers.

You can read Nielsen’s press release here.