After updating my iPhone 3G to 4.2.1, 2-3 week later I found no Wifi bar (greyed WiFi) on my iPhone.
First, I tried to reset network settings on iPhone. Go to Settings -> General -> Network -> Reset -> Reset Network Settings. This worked for temporary only, then greyed WiFI appears again.
Because there’s still no Wi-Fi Address, then I continued to Step Two.
Step Two
On the device, tap Settings > General > Reset > Erase All Content and Settings. Check to see if the device now displays the Wi-Fi address (tap Settings > General > About and scroll down to Wi-Fi Address). But, then my iPhone stuck on apple logo more then 1 day.
then I continued to Step Three.
Step Three
Restore the iPhone or iPod touch software (this erases all data on the device) using iTunes (see article 305744 for additional assistance). It is also failed.
then I continue to Step Four.
Step Four
Downgrade from 4.2.1 to latter version. I downgraded to 4.1, then to 4.0 and finally to 3.1.3 but all did not fix the wiFi problem.
The conclusion, my iPhone may need to be repaired.
Oh poor my iPhone … 
( Anyone can help? I will be very appreciate it.
