Apple has fixed the PDF security hole that allowed websites like JailbreakMe to run a jb directly through the browser, as they announced last week. The latest firmware update is iOS 4.3.4.
JailbreakMe uses a PDF exploit to install Cydia on your iDevice (how to Jailbreak with JailbreakMe). Users who run JBMe are advised to download PDF Patcher 2 from Cydia after they run their jailbreak. Apple isn't just trying to kill your JB fun with this update (well, they are trying to kill the jailbreak) but the bigger issue is that the same exploit could be used to install truly malicious content on an user's iOS device. All they'd have to do is open a PDF with said content.
To update, simply plug your iDevice and launch iTunes. Under your "Devices" list find the device you want to update and click on "Update." iTunes will inform you that a new version is available and will then perform your update.
Update: Apple has information on their website explaining in more detail what's new about the fix: link