Right now, if you visit a web page and load a simple PDF file, you may give total control of your iPhone, iPod touch, or iPad to a hacker. The security bug affects all devices running iOS 3.1.2 and higher.
Update: Initially we thought that this exploit only effected iOS4 devices, but it turns out all iPhones, iPod Touches and iPads running 3.1.2 and higher are susceptible.
The vulnerability is easily exploitable. In fact, the latest one-click, no-computer-required Jailbreak solution for iOS 4 devices uses this same method to break Apple's own security (although in a completely benign way for the user).