Looks like Apple has plugged the PDF exploit in iOS with the now-available iOS 4.3.4 which you can download via iTunes. The PDF exploit was used by JailbreakMe, so if you're planning to use it to jailbreak your iPhone, you'll want to avoid this update.
You can read more about the update here on Apple site, noting that the security updates appear to be in effect all the way back to iOS 3.0, correcting the portions of iOS which were being exploited by an issue in how FreeType handled TrueType fonts. Here's a link to the CDMA explanation of the fix.
The update is for iPad, iPod touch (3rd generation), iPhone 4, iPod touch (4th generation), iPhone 3GS and is a pretty quick download and update.
Here are direct links for each model supported by the update: