Circumvent Apples Mac App store receipt validation.
Install the latest Snow Leopard update(10.6.6). Sign into the new App Store (in your dock). Download Twitter, it’s free. Then go to Applications folder, locate Twitter, right click, Show Package Contents, navigate to Contents folder and copy _CodeSignature, _MASReceipt and CodeResources.
Download Angry Birds ( http://bit.ly/gy9wzk ).. run the dmg file.. drag Angry Birds into the Applications folder. right click, Show Package Contents, navigate to Contents folder and delete _CodeSignature, _MASReceipt and CodeResources. Now paste in the files you copied from the Twitter.app .
Done. Enjoy Angry Birds and any other .app file from the new Mac App Store that you can find online 😉