Minecraft wiki entry for Development_versions, associated text: Rewrote Key Config.
Function and sprint keys can be changed.
By default, sprinting can be activated by holding down the left control key.
In Creative mode, this can also be done while flying.
The original sprinting controls (double-tap forward or double-tap backward while holding down forward) still function.
Smooth mouse, formerly available by pressing F8, will now use F4 by default.