iPhone 3GS 的 S 是 Speed。[1][2]
iPhone 4S 的 S 截至目前未有官方解释。[2][3]
[1] http://www.macworld.com/article/141031/2009/06/iphone3gs.html Para. 3, “The ‘S’ stands for speed,” Phil Schiller, Apple’s senior vice president of worldwide product marketing, told Worldwide Developers Conference attendees during Monday’s conference-opening keynote.
[2] http://www.quora.com/Does-the-S-in-the-iPhone-4S-stand-for-Siri-or-Speed