To the delight of font purists
everywhere, the iPhone system
interface uses Helvetica or a variant
thereof.The original iPhone, iPhone 3G and
iPhone 3GS system interface uses
Helvetica. As first noted by the
always excellent DaringFireball, the
iPhone 4 uses a subtly revised font
called “Helvetica Neue.”
DaringFireball also notes that this
change is related to the iPhone 4
display rather than the iOS 4
operating system and older iPhone
models running iOS 4 still use
Helvetica as the system font.iPod models released prior to the
iPhone use either Chicago, Espy Sans,
or Myriad and use Helvetica after the
release of the iPhone.
From http://www.everyipod.com/iphone-faq/iphone-who-designed-iphone-font-used-iphone-ringtones.html
For iOS9 it has changed to San Francisco. See http://developer.apple.com/fonts for more info.