Obtenido de http://www.integratingstuff.com/2012/02/25/some-ios-development-tips/ Macro para programar apps universales y ejemplo

UIUtil.h

#ifdef UI_USER_INTERFACE_IDIOM()
#define IS_IPAD() (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
#else
#define IS_IPAD() (false)
#endif

#import "UiUtil.h"

@implementation UiUtil

+(NSString *) getNibNameFrom: (NSString*) nibBaseName{
    if (IS_IPAD()){
        NSMutableString *nibForiPad = [NSMutableString stringWithString: nibBaseName];
        [nibForiPad appendString:@"-iPad"];
        return nibForiPad;
    }
    else {
         return nibBaseName;
    }
}

+(NSString *) getBackgroundNameFrom: (NSString*) backgroundBaseName{
    if (IS_IPAD()){
        NSMutableString *backgroundForiPad = [NSMutableString stringWithString: backgroundBaseName];
        [backgroundForiPad replaceOccurrencesOfString:@".png" withString:@"-iPad.png" options:0 range:NSMakeRange(0, [backgroundForiPad length])];
        return backgroundForiPad;
    }
    else {
        return backgroundBaseName;
    }
}

+(CGFloat) getFontModifier{
    if (IS_IPAD()){
        return 2;
    }
    else {
        return 1.0;
    }
}

@end