According to Apple, the update -- which applies to the iPhone, iPhone 3G and iPhone 3GS -- has the following improvements:
Resolves sporadic issue that may cause iPhone to not wake from sleep
Resolves intermittent issue that may interrupt cellular network services until restart
Fixes bug that could cause occasional crash during video streaming
The update, which weighs in at over 200MB, can be downloaded through iTunes. A corresponding update for the iPod touch was also released.
A month ago, Apple released iPhone OS 3.1, which debuted a number of new features, including a Genius recommendation system for software in the App Store. It also added ringtones from record labels, directly into the iTunes application on the phone.