According to Jesus Diaz at gizmodo.com, downloading a simple PDF file can give hackers access to your iPhone, iPod touch, or iPad.
The security hole is applicable to all iOS 4 devices.
According to Diaz, "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...It just requires the user to visit a web address using Safari. The web site can automatically load a simple PDF document, which contains a font that hides a special program. When your iOS device tries to display the PDF file, that font causes something stack overflow...The result is that, without any user intervention whatsoever, that program can do whatever it wants inside your iPhone, iPod touch, or iPad."
For the complete story, see Apple Security Breach Gives Complete Access to Your iPhone.