iOS 4 got a software update (it's now 4.0.2) that fixes the PDF security flaw. This security flaw allowed hackers to write arbitrary code and could have been accessed from the mobile browser.
The closing of this flaw means two things. One is that hackers can no longer break into the device, be it the iPhone, iPod Touch or iPad, and write malicious code. The second is that the device can no longer be jailbroken. The iOS 4jailbreak that was released two weeks ago worked on this security flaw.
SOURCE: GSM Arena