Today Apple released the latest iPhone firmware update, 3.1.2, aimed at fixing several bugs infesting the current operating system, including: a remedy for problems with Sleep mode, issues with the cellular network, and crashes during video streaming.
If you ever want to jailbreak your iPhone: do not perform this update. I repeat: do not update to 3.1.2.
The iPhone Dev-Team just issued a warning on their blog saying:
"If you care about your jailbreak and unlock, don’t update your device - 3G and 3G(S) owners should pay particular attention to this warning. PwnageTool and redsn0w are not yet compatible with 3.1.2. There is no estimated release time for compatible tools (please don’t bug us about this)."
If you have no intention of jailbreaking, go ahead and download the firmware. Keep in mind, though, that it might take a while as the update is over 240 megabytes.
