Smartphone sales fell in the U.S. for the first time during Q3 by Philip Elmer-DeWitt @FortuneMagazine October 24, 2011, 2:14 PM EDT E-mail Tweet Facebook Google Plus Linkedin Share icons Analyst sees “ominous signs” for Android vendors. Says Q4 may be the quarter ‘droid users switch to Apple’s iPhone 4S Click to enlarge. Source: Morgan Keegan Drawing from AT&T’s T and Verizon’s VZ reports for the September quarter, Morgan Keegan’s Tavis McCourt sees what could be bad news for the makers of Android phones and good news for Apple AAPL . In a note to clients issued Monday, he writes (I quote): After analyzing AT&T and Verizon results, we believe it is likely that smartphone sales may have declined in Q3:11 both sequentially and Y/Y for the first time ever in the US. Obviously, a big part of this was the impact of the iPhone 4S being delayed until Q4, but there are also some ominous signs for Android vendors in these results given that there were a number of Android launches in the quarter at AT&T and Verizon, and yet Android sales likely declined sequentially and had only modest growth Y/Y. We believe slowdown in Android/Blackberry/[Windows Phone] sales at AT&T/Verizon likely indicates a reasonable number of customers choosing not to upgrade in Q3, but rather waiting to switch to the iPhone in Q4. The strong initial sales figures for iPhone 4S would seem to back up this assumption. We believe this data also speaks to the fact that the “high end” adoption of smartphones in the US is nearly complete (45% of AT&T/VZ postpaid subs now on smartphone plans), and we expect more carrier experimentation with lower priced plans to expand adoption (we note Verizon’s recent embrace of Blackberry Social plans is a strong indication of this). Although Samsung may have since overtaken it, Apple was still the world’s No. 1 maker of smartphones, with 18.1% market share in Q2, according to Gartner, the manufacturers of phones running Google’s GOOG Android operating system together account for 43.4% of worldwide smartphone sales.