Apple has released iOS 4.3.4 update (iOS 4.2.9 for Verizon iPhone 4) that patches the vulnerability found in the Safari browser which has been used to jailbreak iOS deices like the recent JailBreakMe.com jailbreak and which could also allow execution of malicious code to infect users devices. The update also fixes the PDF exploit found in the earlier version. If you have a jailbroken device and choose to update to iOS 4.3.4, you must note that the update will break your current jailbreak.
The iOS 4.3.4 update is available for iPhone 4 (GSM), iPhone 3GS, iPad 2, iPad, iPod Touch (3rd and 4th gen). The same is also available as iOS 4.2.9 update for the Verizon CDMA iPhone. The iOS 4.3.4 update bears the Build 8K2, while the iOS 4.2.9 is having the Build 8E501.
However, as is the case always with the iOS releases, iOS 4.3.4 has also been jailbroken within hours of the release of the update, courtesy custom Pwnage Tool. The complete tutorial on how to jailbreak iOS 4.3.4 is available at Redmond Pie. This is a tethered jailbreak and is currently works on iPhone, iPod Touch and the first gen iPad but not the iPad 2.