A zero-day PDF vulnerability has been found by Comex from the website JailbreakMe.com and is currently being used to jailbreak current iOS devices. However, the vulnerability, present in even the most recent iOS version, could be used for much more nefarious purposes.
The real kicker in this story is that once you use the exploit to jailbreak your device, you can install a patch to fix the vulnerability. For the time being, then, jailbroken devices are more secure than stock iPhones.
Apple is already aware of the issue and is said to be working on a fix, but jailbroken users still had one first. Even if this occurrence is not a permanent reason to jailbreak, it certainly undermines the idea that a stock device is always more secure than a jailbroken one.[Intego]