As iFonts will show you, iOS 5.1 brings new fonts to iPhone and iPad. AppleGothic has been replaced with Apple SD Gothic Neo Bold and Apple SD Gothic Neo Medium.
According to iLessons iLearned, these are Korean fonts. The Korean letters look much better in these fonts than the English ones, so if you’re going to use this font, I’d recommend writing something in Korean. :)
Check out these new fonts on your iPhone with iFonts, the black-and-whitest app for viewing what fonts are included in iOS.