There are some people who complain of the Galaxy Nexus's slow responding proximity sensor. I've experienced the same thing. It doesn't matter if you have the CDMA or the GSM model.
The problem is when you hold it up to your ear, the screen is still on for about a second. This causes your face or ear to dial a few numbers before the screen finally turns off. This causes confusion for people on the other line, or cause havoc on automated systems.
However, I found out this wasn't a hardware issue. In fact, it's by design, and it can be remedied in software. I found this out when I used a 3rd party app called GrooVe IP, which is a dialer for Google Voice. It lets you make calls using your Goolge Voice number via the internet (WIFI or 3G). As a dialer, it also turns off the screen when you hold the phone up to your ear.
I noticed I didn't have any of the ear-dialing problems. So to test, I placed the phone on a flat surface, dialed, and put my hand over the proximity sensor. To my surprise, when using the stock dialer, the screen did not turn off. I knew it did, because when I pulled away from my ear, I saw a black screen turning back on. As a hunch, I placed the phone in a vertical position and covered the proximity sensory again. This time, the screen turned off!
What I found out was the stock dialer needed two requirements before it shuts off the screen. First, the phone needs to be in a vertical position (using your acelerometer sensor), then proximity sensor needs to sense your ear or face. The order is important because if you place your hand over the proximity sensor first (the screen does not turn off), then put it in a verticial position, the screen will not turn off.
To prove this is a software and not a hardware issue, I used GrooVe IP and dialed a number. Even on a flat surface, as soon as my had goes near the proximity sensor, the screen turns off right away.
So it isn't an issue of a slow responding proximity sensor (a hardware issue). It's an overly complicated design that resulted in this behavior. I guess it was designed this way so that the screen stays on if you accidentally covered the sensor with your hand while holding it, but that rarely happens. Even the iPhone does not do this.
I hope Google fixes this in the next update. We haven't recieved the Android 4.0.3 update yet. I hope the fix is in the works. However, there aren't a large number of complaints, so it may affect a small amount of people. This might mean that word hasn't reached the Googleplex yet.
Hopefully this changes it. Of course, who reads this dinky little blog?