Non-opaque application windows in iOS 7, with optional blur. Shows the user's wallpaper under the app, with Parallax if supported.

UIBackgroundStyle-Test.entitlements

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>com.apple.springboard.appbackgroundstyle</key>
	<true/>
</dict>
</plist>

UIBackgroundStyle-Test.m

#import "UIBackgroundStyle.h"

// Your binary will need the (private) entitlement "com.apple.springboard.appbackgroundstyle"

[application _setBackgroundStyle:UIBackgroundStyleDarkBlur];

UIBackgroundStyle.h

#import <UIKit/UIKit.h>

typedef NS_ENUM(NSUInteger, UIBackgroundStyle) {
    UIBackgroundStyleDefault,
    UIBackgroundStyleTransparent,
    UIBackgroundStyleLightBlur,
    UIBackgroundStyleDarkBlur,
    UIBackgroundStyleDarkTranslucent,
    UIBackgroundStyleExtraDarkBlur,
    UIBackgroundStyleBlur
};

@interface UIApplication (UIBackgroundStyle)
// Requires "com.apple.springboard.appbackgroundstyle" entitlement.
-(void)_setBackgroundStyle:(UIBackgroundStyle)style;
@end