For the first time since it launched last October, Apple's iPhone 4S was not the top selling smartphone in the U.S., as the newly released Samsung Galaxy S III took the top spot in the month of August.

Samsung's lead is expected by analyst Michael Walkley with Canaccord Genuity to be short-lived, however, as Apple is widely expected to unveil its next-generation iPhone next week. Walkley said in a note to investors on Tuesday that his checks with U.S. carriers found that iPhone sales have been soft ahead of the launch of Apple's next iPhone, allowing Samsung's Galaxy S III to take the top spot.

The data from carriers indicated that many U.S. customers are holding off on buying a smartphone until Apple's next iPhone debuts. The Galaxy S III was the top selling handset at three of the four major carriers in America: Verizon, Sprint and T-Mobile. But the iPhone 4S remained the top seller at AT&T, according to Walkley's checks.

In fact, at Verizon the iPhone 4S was bumped down to third place in August, finishing behind the Motorola Razr Maxx as well. The iPhone 4S took second place at Sprint, while it is not available through T-Mobile.

"We believe this is the first month since the iPhone 4S launched in October 2011 that it was not the top selling smartphone in the U.S. market," he wrote. "However, our checks indicated strong consumer interest and likely demand for the iPhone 5, and we believe Apple will return to strong number one smartphone share in the U.S. post the iPhone 5 launch."