I tested this on two C8 Nexus(es). I don't guarantee if it will work for you or not but it looks like it fixed the issue for me. The only downside of this fix is that you have to use power button in order to lock your screen while in call and after the call you have to unlock it manually (This isn't a big issue in front of this issue, right?). Oh and by the way, it works on Android JellyBean version 4.2 and is untested on previous versions of Android.
So let's start:
1. So for the first step, you have to root your phone and flash Clockworkmod recovery into your Galaxy Nexus.
(WARNING: Flashing recovery and rooting leads to loss of all the data placed in your sdcard, so make sure your backup beforehand because it's better than crying later)
2. Download this zip and place it in your phone's sdcard: http://www.mediafire.com/?ir6z384vutepdmu
(To revert you can flash this (PLEASE NOTE THIS IS Version 2 and please use this version to revert successfully previous version had some flaws in it): http://www.mediafire.com/?d4n1fx3cj446k1l)
3. Power off the phone, after that press Volume up + Volume down + Power button to go into bootloader mode.
4. After entering into bootloader mode press Volume up two times and "Recovery Mode" should come up, press power button and the phone will restart into Recovery mode.
5. Select "install zip from sdcard" then press "choose zip from sdcard" then "0" then tap on the "ProximityFix.zip" (which you downloaded in step 2) and in last press "Yes".
6. After flashing of the zip is done tap on the "+++Go back+++" and then "reboot system now".
Voila you have a proximity sensor that no more interferes into calls or anything else!
What I did?
Basically I just Hexedit'ed "sensors.tuna.so" found inside "/system/lib/hw/" and changed "proximity" to "proximits".
What it does?
The stock "sensors.tuna.so" gets replaced by the modded "sensors.tuna.so" in order to completely disable the proximity.
A very good friend of mine (zurchpet) from IRC for making edify script changes for the above supplied zip.
Alexander T. for figuring out production date.