After the release of the untethered jailbreak solution this morning, saurik also released an update for MobileSubstrate, the plugin based on which most of the tweaks available in Cydia work. This new update brings compatibility with iOS 4.3.1 and will be necessary to run the new version of ultrasn0w as soon as it is released by the Dev Team. If you have iOS 4.3/4.3.1 you will have to install this update directly from Cydia, otherwise some tweaks will not work properly when you install them.
Jailbreak tutorials for iOS 4.3.1 can be found here: redsn0w, Pwnagetool.
UPDATED: The update does not affect the plugin itself but its other dependencies, namely libxml and sqlite.
