According to David Pogue in the New York Times, the accessibility features like VoiceOver introduced with the iPhone 3GS make it one of the easiest smartphones for the blind and visually impaired:

First, you tap something to hear it. You can tap icons, words, even the tiny status icons at the top of the screen. As you go, the voice tells you what you’re tapping. “Messages.” “Calendar.” “Mail — 14 new items.” “45 percent battery power.”

Once enabled in the settings, Pogue details how new gestures like double-tap to launch, two-finger upward flick to read, three finger flick to scroll or go to next/previous screen, and three finger triple tap to turn the screen off and save power.

You can also use "the rotor", a two finger circle, to change the way other gestures work -- proofreading text character by character rather than word by word, for example.

Supported languages included Bahasa Indonesian, Chinese (Cantonese), Chinese (China), Chinese (Taiwan), Dutch, English (US), English (UK), English (Australian), Finnish, French (Canada), French (France), German, Greek, Italian, Japanese, Korean, Norwegian, Polish, Portuguese (Brazil), Portuguese (Portugal), Romanian, Russian, Slovak, Spanish (Mexico), Spanish (Spain), Swedish, Thai, Turkish.

For more on VoiceOver, see Apple's overview.