Is anyone having issues with their screens being non responsive. Really annoying when I cant answer incoming calls. Intermittent issue (1-2 times per day). In some cases lasts for 20 minutes. A friend of mine with another 3GS is having the same troubles.
1. Reset phone - press both home and sleep/wake buttons until the Apple logo appears (10+ seconds)
2. Remove any screen protector or film on the phone.
3. Restore phone in iTunes using a backup.
4. Restore as a new iPhone, without using a backup.
If #4 doesn't help, there may be a hardware problem with the screen that needs evaluation by Apple.