The slowdown in iPhone sales in advance of the October release of iPhone 4S is reflected in the report on mobile phone shipments for the third quarter of 2011 issued Tuesday by Gartner, Inc.

Apple (AAPL) shipped 17 million iPhones in the quarter — up 21% year over year but down nearly 3 million units from the previous quarter. Samsung, meanwhile, shipped 24 million smartphones in the quarter, overtaking Nokia (NOK) to become, for the first time, the world’s largest manufacturer of smartphones.

The quarter also marked the first time Google’s (GOOG) Android operating system accounted for more than 50% of the global smartphone shipments. Apple’s iOS, meanwhile, fell from a 16.6% share in Q3 2010 to a 15% share one year later. (See, however, Android sells the smartphones; Apple makes the money.)

Regarding the iPhone’s prospects, Gartner’s press release had this to say:

Gartner believes Apple will bounce back in the fourth quarter because of its strongest ever preorders for the iPhone 4S in the first weekend after its announcement. Markets such as Brazil, Mexico, Russia and China are becoming more important to Apple, representing 16 percent of overall sales and showing that the iPhone has a place in emerging markets, especially now that the 3GS and 4 have received price cuts.

Below: Gartner’s spreadsheet of global shipments of all mobile phone sales, smart and not-so smart.