For detecting retina display on all devices including the new iPad

    +(BOOL)isRetinaDisplay {
    // since we call this alot, cache it
    static CGFloat scale = 0.0;
    if (scale == 0.0) {
        // NOTE: In order to detect the Retina display reliably on all iOS devices,
        // you need to check if the device is running iOS4+ and if the 
        // [UIScreen mainScreen].scale property is equal to 2.0. 
        // You CANNOT assume a device is running iOS4+ if the scale property exists,
        // as the iPad 3.2 also contains this property.
        // On an iPad running iOS3.2, scale will return 1.0 in 1x mode, and 2.0
        // in 2x mode -- even though we know that device does not contain a Retina display.
        // Apple changed this behavior in iOS4.2 for the iPad: it returns 1.0 in both
        // 1x and 2x modes. You can test this yourself in the simulator.
        // I test for the -displayLinkWithTarget:selector: method on the main screen
        // which exists in iOS4.x but not iOS3.2, and then check the screen's scale:

        if ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)] && 
            ([UIScreen mainScreen].scale == 2.0)) {
            scale = 2.0;
            return YES;
        } else {
            scale = 1.0;
            return NO;
        }   

    }
    return scale > 1.0;
}

Credit to: Adriano Paladini http://developer.appcelerator.com/question/133826/detecting-new-ipad-3-dpi-and-retina

- (NSString *) platform  
{  
    size_t size;  
    sysctlbyname("hw.machine", NULL, &size, NULL, 0);  
    char *machine = malloc(size);  
    sysctlbyname("hw.machine", machine, &size, NULL, 0);  
    NSString *platform = [NSString stringWithCString:machine];  
    free(machine);  
    return platform;  
}  

- (NSString *) platformString  
{  
    NSString *platform = [self platform];  
    if ([platform isEqualToString:@"iPhone1,1"]) return @"Original iPhone";  
    if ([platform isEqualToString:@"iPhone1,2"]) return @"iPhone 3G";  
    if ([platform isEqualToString:@"iPhone2,1"]) return @"iPhone 3G[S]"; 
    if ([platform isEqualToString:@"iPhone3,1"]) return @"iPhone 4";   
    return @"Unknown";  
}  

Checking the scale property is not sufficient, as on iPad 3.2 in 2x mode, the scale property exists and will return 2.0, but we know that device does NOT have a Retina display.

I've created at category on UIScreen to do this. For a more detailed explanation, see my answer to Detect Retina Display. Here's the code:

@interface UIScreen(ZBScreenRetinaAdditions)

// Returns YES if this is a Retina display.
- (BOOL)zb_isRetina;

@end

@implementation UIScreen(ZBScreenRetinaAdditions)

- (BOOL)zb_isRetina {
  return [self respondsToSelector:@selector(displayLinkWithTarget:selector:)] && (self.scale == 2.0);
}

@end

Usage example:

if ([UIScreen mainScreen] zb_isRetina) {
  // Retina display
}

just adding my 2 cents :

I see what you are doing here, but binding this to a specific value like 2.0 is good for the moment, but what if for example next iPad got resolution bump like x1.5 ? For me anything that goes above 1.0 is kind of higher-than-normal display so I will load hi-res graphics. Does not matter much then if that is iPad,iPhone ....

import "UIScreen+Retina.h"

if([UIDevice currentDevice].userInterfaceIdiom == UIUserInterfaceIdiomPad){
    //IPAD        
    if ([[UIScreen mainScreen] isRetina]) {
        // IPAD 3 - Retina display
        bannersGetPromoServerRequest.size = kXML_API_IMAGESIZE_IPAD_HIGHRES;            
    }else{
        //iPAD 1/2
        bannersGetPromoServerRequest.size = kXML_API_IMAGESIZE_IPAD_LOWRES;        }
}else{
    //IPHONE
    if ([[UIScreen mainScreen] isRetina]) {
        // IPHONE 4/4s/5 - Retina display
        bannersGetPromoServerRequest.size = kXML_API_IMAGESIZE_IPHONE_HIGHRES;

    }else{
        //IPHONE (3.x)
        bannersGetPromoServerRequest.size = kXML_API_IMAGESIZE_IPHONE_LOWRES;

    }
}