There have been several reports about additional problems after upgrading to iPhone OS 3.0 or even the iPhone 3GS and Cnet has come up with a few procedures which will hopefully solve a few of the problems.
Reset your iPhone by holding down the Sleep and Home buttons until the Apple logo appears…and to restore your iPhone…In iTunes, click the Restore button under the Summary tab. Restoring the phone will erase contacts, calendars, photos, and other data on the phone, but will restore automatically backed-up information including text messages, notes, call history, contact favourites, sound settings, widget settings, etc.
However additional problems have still persisted namely sound issues such as distortion and an audible whine when recording videos on the iPhone 3GS, and there doesn’t seem to be a fix for that yet. Another problem is wrong or changing icons, and the only fix found is to reinstall the affected application and cross your fingers.