We may be getting a glimpse of the answer to that question now, and it appears to vary by country. According to Kantar Worldpanel ComTech, iPhone sales in the US, UK, and Australia over the last 12 weeks have made up for lower than normal gains earlier in the year. This seems to follow the expected pattern of existing iPhone customers upgrading.
However, in Germany and France the iPhone isn't doing nearly as well. Not surprisingly this appears to be closely related to the success of Samsung's Galaxy S II. In fact, in Germany the Galaxy S II was the top selling smartphone during the same period.
Keep in mind, the Galaxy S II was available throughout Europe months before it arrived in the US. In fact, most US carriers didn't get it until around the time of the iPhone 4S launch.
While it seems unlikely Apple sales wouldn't have rebounded, it does beg the question of how US perception of the smartphone market might have changed had it been available earlier.