While two-thirds of Apple's (AAPL 0.53%) revenue now comes from international markets, that doesn't mean the party's over back home. Developed markets like the U.S. do have greater smartphone saturation than emerging markets, but there are still plenty of potential first-time smartphone buyers to go after. Domestically, 30% of mobile phone buyers have yet to buy their first smartphones. At the same time, Apple generates reliable recurring revenue from a constant string of upgrades and replacements. In the following slideshow, there are four charts that illustrate just how much opportunity Apple still has in the United States.