I took my iPhone into a genius appointment yesterday. They took it into the back area and upon returning informed me that they installed a "patch," I couldn't restore from my previous back-up, and that it was a known problem.
Apparently this is a software issue that only affects those upgrading their iPhones (the iPhone 4 doesn't like something in the back-up), not brand-new users.
All my apps/ settings were intact and my version number is the same as the regular shipped build.
I was told to restore my iPhone fresh (which would delete the installed patch, no?) and not to restore from back-up, which is possibly the fix itself. I'm going to do it now and will report back on the results... did you upgrade from a previous iPhone and restored from a previous back-up?
Just tested against a co-workers by placing on a flat table and moving fingers to cover up the sensor... mine seems to work consistently within about an inch of cover-up, his is hit-or-miss even while physically covering / pressing on the sensor.