For those of you who want the screen off effect from Nexus series, but don't want to root your phone, i have found a small but great app, that can do just that, plus it has like 10 screen off effects and you can also use sound effects for lock / unlock screen.
You can put the shortcut on your home screen or even in the main dock for fast access, this way you can lock & close your screen in just a flash. Also, you can make a shortcut on your Lock Screen and you got all your phone covered.
Give it a try, here it is:
Screen Off and Lock (Donate) v1.14 - mirror
If you like this app, be sure to Rate it in the Market and buy / donate to support the developer. Google Play
Update: Currently working properly Samsung Galaxy S3 GT-i9300.