The update also reportedly resolves an issue where third-party applications would not launch in some instances. It also addresses a bug that could cause applications to crash when using the Japanese Kana keyboard.
iPhone OS 3.1.3 also includes a number of security updates related to CoreAudio, ImageIO, Recovery Mode and WebKit. Full details are available in the support document on Apple's Web site.
The update for the iPhone and iPod touch operating system is available for download through iTunes.
The last update, iPhone OS 3.1.2, arrived last October. It fixed an issue that would cause the iPhone to not wake from sleep, and resolved a glitch that could interrupt cellular network services until restart.
Last week, iPhone OS 3.2 was revealed as part of the new multi-touch iPad. For now, the operating system upgrade is for the iPad hardware only, which is scheduled to launch before the end of March.
iPhone OS 3.2 offers a number of enhancements over the current operating system, including a home screen that can be viewed either horizontally or vertically. It is not yet known what additional features in the iPad version of the iPhone OS could show up in the iPhone or iPod touch.