For any new products being released to commercial market, it is common to see some minor bugs which happened similarly to the newly launched iPhone 4. Other than the recently mentioned poor reception and yellowish spots issues, seems that some early users have discovered another issue related to its proximity sensor.
Proximity sensor’s functionality is simple but yet is quite useful for those that always accidentally touch its sensitive screen whenever the phone is brought closer when communication is going on. However, several online users have initiated discussion in Apple Support Discussion forum citing that some of their iPhone 4’s proximity sensors are not working correctly. Even though the iPhone 4 screen will go blank when the sensor is activated, but in actual fact users can still toggle some of its functions unintentionally, unlike those that being prohibited in its predecessor 3G and 3GS. For iPhone 3G and 3GS models, the proximity sensor is a piece of flex circuitry that resided near to the speaker portion and it can easily get changed whenever it is malfunction.
Apple Inc has not acknowledged with official feedback yet but potential quick workaround that worth to try out include a full restore (do remember to do a backup first) and see if it helps to eliminate the issue. This is at least better than waiting for a new replacement that may take time due to overwhelming response and limited supply.