This is an Apple bug. It was introduced in iOS 7.0.3 and has not yet been fixed as of iOS 7.0.4. It appears to be fixed in the developer preview of iOS 7.1. Here is code (provided by Apple in the dev forums) to workaround the issue:

#import <CoreText/CoreText.h>

CGFloat size = 14;
UIFont *font = [UIFont fontWithName:@"HelveticaNeue-Italic" size:size];
if (font == nil && ([UIFontDescriptor class] != nil)) {
    font = (__bridge_transfer UIFont*)CTFontCreateWithName(CFSTR("HelveticaNeue-Italic"), size, NULL);
}

It is also worth noting that in the current version of Xcode (5.0.1 (5A2053)) this font is not listed as an option in the Font drop down list in Interface Builder. So if you previously configured a label with this font you will notice that the ui is confused and the label ends up being assigned some other font and size at runtime (see ui screencap below). For labels configured in storyboards/xibs you will need to reset the font in code.

For reference here is the discussion of the issue in the dev forums.

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