How do i correct my far-too-sensitive proximity sensor?
Z-Device test may show your sensor output: https://play.google.com/store/apps/details?id=zausan.zdevicetest

Possible reasons:


some aftermarket display protectors may cause that
an earlier Android release only switched the screen off via proximity sensor when held vertically. This would have prevented your described problem. (See http://www.guytechie.com/articles/2012/1/22/galaxy-nexus-slow-proximity-sensor.html). Maybe this behaviour has been reverted.


If you updated just before noticing this problem, you might have to switch back or wait for another update to fix that again. Installing a 3rd party update like Cyanogenmod9 might also fix your problem.