Apple has just released an iOS security update that patches a vulnerability with certificate validation. The update, version 4.3.5 (build 8L1), is available to all devices compatible with the previous 4.3.4 update and is recommended to all. Sadly, there is no OTA in iOS 5 to patch the vulnerability with only a small download, but instead you must download the entire ~600 MB file instead.
No one knows if this patches a jailbreak exploit or not, so it is advised to wait until you hear from a member of the iPhone dev team before updating – if you value your jailbreak. Head past the break for direct links to the update.