4 in 10 U.S. phones are now smartphones

Last year Nielsen predicted that by the end of 2011, smartphones will overtake feature phones in market share in the U.S. That may be a little optimistic, but the advance of smartphones continues unabated, with smartphones now representing 40 percent of all mobile phones in the U.S., according to Nielsen.

Looking at the three-month period ending in July, Nielsen said that feature phones have slipped to 60 percent, down from 62 percent in the three-month period ending in May. At this pace, smartphones won’t cross over into the majority probably until sometime in 2012, if they maintain their growth. And that’s if you believe the Nielsen numbers. ComScore recently reported that smartphone penetration was about 35 percent. But regardless of which source you believe, smartphones are still showing strong momentum and will increasingly be a tool used by mainstream cell phone users.

Android (s goog) continues to be the top performer with 40 percent share in the U.S., according to Nielsen, up 1 percent from the second quarter, while iOS (s aapl) remained steady at 28 percent. BlackBerry (s rimm) fell 1 percentage point to 19 percent over the same period while Windows Phone 7 and Windows Mobile (s msft) combined for 8 percent, down 1 percentage point.

Android remains the top choice among “innovators,” or the earliest adopters, with 40 percent saying their next smartphone will be an Android device, compared with 32 percent who said iOS. But among early adopters and early and late majority users, the difference between the two is much smaller, with iOS and Android trading the top spot.

Interestingly, among late adopters, 30 percent are undecided about their next smartphone choice. These are likely feature phone users who are still not sure what they want to buy if they make the jump up. It’s this group that’s going to be hard fought over, because they’re not in any one camp yet, similar to undecided voters in an election, Nielsen points out. But as smartphones eventually overtake feature phones, the race will be on to sew up this last group.