For all those of you who have used the JailbreakMe.com method of jailbreaking then you took advantage of a PDF exploit in the Safari application. This is good that you can jailbreak your device so easily but it also means anyone with the right know how can run code on your iDevice. This means that if in the wrong hands it could be used for malicious purposes.
The iPhone Dev-Team or Apple will eventually patch the PDF hole, probably the latter. For now you need to make sure your device is protected so that you don’t fall into any malicious traps. You need to install a Cydia package called PDF Loading Warner, this package displays a pop-up when your device is about to open a PDF. It won’t stop this exploit but it will let you choose not to open a PDF that might be malicious. It has caused some issues with iBooks when opening PDF files as you have to dismiss the pop-up numerous times. However it’s a small price to pay for such an easy jailbreak.