Few hours ago iPhone software update version 2.2.1 arrived in iTunes. When you start iTunes and hit the check updates button, the following window will pop up.

NOTE: whether you have a jailbroken and or unlocked iPhone, it is wise not to install it yet. The effects are currently unknown.
[UPDATE] plus keep monitoring “Hold Your Horses!” http://is.gd/hqIz [dev-team blog]
[UPDATE 30 Jan 2009]

Orginal post by Waku_Waku