Today Apple released iOS versions 4.3.4 and 4.2.9 to close the PDF exploit used in Jailbreakme.com (and supposed intended other malicious exploits).
If you are jailbroken, do not update your device.
If you have not already done so, save your SHSH Blobs for 4.3.3 / 4.2.8 as soon as humanly possible as Apple is likely to stop signing the previous firmware shortly. You can find the tool to grab your SHSH files here – http://thefirmwareumbrella.blogspot.com/.
