Apple has released an update for the iPhone, bringing the version number up to 3.1.3. It brings a few minor new features, a whole host of bug fixes, and a handful of security patches which close up the holes that jailbreakers use to get inside the handset.

The biggest new feature seems to be that the battery gauge indicator has got more accurate. Yup, the patch really is that minor. It also fixes a problem where third party apps wouldn't launch, and a potential crash when apps used the Japanese Kana keyboard.

Nasty MP4 and TIFF files have been protected against with patches from CoreAudio and  ImageIO, and WebKit has been tweaked to better protect against HTML Mail messages and dodgy FTP servers. There's also better handling of USB control messages in recovery mode.

As a result, it seems like the most popular jailbreaking tools - Redsn0w, Blackra1n and PwnageTool - are all incompatible with this update, and their creators are warning jailbreakers not to install the patch, especially if you've got a 3G or a 3GS.

The download is a whopping 291MB, and is available now by plugging your handset into iTunes and letting it do its thing. Make sure you back up first, just in case something goes wrong.