UPDATE: just added a sample app that allows you to control the window blur with a slider.
Here is a working example of an NSWindow with a blurred glass background similar to Vista and iOS 7. I also attached an image showing the blur cranked up even more.
It uses a private API so this means it probably can't be used for apps that are destined for the app store. Fun to create and play with though.
You can get the working Xcode project at https://github.com/brimelow/Cocoa-Glass-Window. It was built using Xcode 5 DP4 so you may need to copy the files into a new project if you're still on Xcode 4.
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