Here is some update for supporting iOS 7. It has Dynamic Font Size now.
For any and all apps that support “Dynamic Type,” users can select a font size in iOS 7 that works system wide, simply by visiting the "General" section under "Settings" and selecting "Font Size."
UIFont *dynamicFont = [UIFont preferredFontForTextStyle:UIFontTextStyleBody];
And constants list, detailed explanation is here
NSString *const UIFontTextStyleHeadline;
NSString *const UIFontTextStyleSubheadline;
NSString *const UIFontTextStyleBody;
NSString *const UIFontTextStyleFootnote;
NSString *const UIFontTextStyleCaption1;
NSString *const UIFontTextStyleCaption2;