SIM Failure on iPhone 4

My iPhone 4 was accidentally thrown a few centimetres away on my desk, then few hours later I saw “SIM failure” on iPhone 4. Not sure if it was caused by this incident, because my iPhone 2G and iPhone 3G were dropped alot and nothing wrong with it. However, I only see this SIM failure after it was softly dropped above my desk.
This problem is solved after I restored the iPhone 4 firmware to iOS 4.0.1. Rarely it shows No Service and loose the signal bar and shows SIM failure. But it was working fine after I restored its firmware once again.