ImageIO in Apple iOS before 4 on the iPhone and iPod touch allows remote attackers to execute arbitrary code or cause a denial of service (memory corruption and application crash) via a crafted JPEG image. Per: http://lists.apple.com/archives/security-announce/2010/Jun/msg00003.html
'Installation note: These updates are only available through iTunes, and will not appear in your computer's Software Update application, or in the Apple Downloads site. Make sure you have an Internet connection and have installed the latest version of iTunes from www.apple.com/itunes/ iTunes will automatically check Apple's update server on its weekly schedule. When an update is detected, it will download it. When the iPhone or iPod touch is docked, iTunes will present the user with the option to install the update. We recommend applying the update immediately if possible. Selecting Don't Install will present the option the next time you connect your iPhone or iPod touch. The automatic update process may take up to a week depending on the day that iTunes checks for updates. You may manually obtain the update via the Check for Updates button within iTunes. After doing this, the update can be applied when your iPhone or iPod touch is docked to your computer. To check that the iPhone or iPod touch has been updated: * Navigate to Settings * Select General * Select About. The version after applying this update will be "4.0 (8A293)" or later.'