The new Jailbreakme exploit which allows users to simply visit a website to jailbreak their iPhones used a pdf exploit to do so. Well when Apple found out about the exploit they had a patch out in record time, like a couple weeks.
Honestly Apple does not usually issue patches or fixes this fast, they usually take months. Apple moving this fast to patch an exploit may mean that they are trying harder to stop jailbreakers and trying to stay ahead of the hackers. More information about the exploits can be found in our source.