Proximity Sensor on Galaxy Nexus

So I finally got my Galaxy Nexus and I love it- however one thing I've noticed is that when I'm making calls the screen doesn't automatically dim to black when I hold it up to talk (at least I haven't noticed it happening).. this is a feature I have come to know and love on my original Galaxy S phone. Was this feature exclusive to TouchWiz?

I used to love getting prompts to "press 1 for..." only to bring the phone down from my ear and have the screen magically turn on.. it worked really well. I believe this uses the on-board proximity sensor but I haven't noticed any features that take advantage of it on the Galaxy Nexus. It makes sense that if I'm talking on the phone I don't want the display turned on and wasting battery life.

Anyone know how to engage the proximity sensor on the Galaxy Nexus or what apps take advantage of this?

Edit: Maybe it is working on my device and I just haven't noticed it.. either way it'd be great to learn about more apps that might use this. Thanks for the responses, they have been helpful.