Lately most of the news concerning iPhone 4 issues have been focused on the "death grip" and reception problems. The iPhone 4 Proximity Sensor is a growing concern as well. The device's proximity sensors designed to sense when the phone is held up to the user's face and deactivate the device's screen, thus preventing unintended activation of buttons on the display. According to MacWorld here's the solution:
Many affected users report that either a Settings reset or a hard reboot fixes their problem. One Apple support thread recommends that users open the Settings app, then go to General -> Reset, and choose "All Settings" at the top (note: this will not erase your apps, media, or other data -- that's what the "Erase All Content and Settings" button just below this option is for. "Reset All Settings" just reverts any Settings preferences you've customized back to their factory defaults). One Twitter follower, Brian Partridge, reported that AppleCare told him to just use Reset -> Reset Network Settings, not all settings, and it worked.
After reading this article, I noticed the problem over the last few days and it has become quite annoying. Let us know if this fix works for you.
Source: MacWorld & Macrumors.com