Enter an instance such as Stockade and enter combat with a mob, then exit the instance .. or simply /afk out of a BG while in combat. You will notice for a moment that InCombatLockdown() is true then it will switch to false but PLAYER_REGEN_ENABLED never fires. This could simply be a semantic problem, but if so is there an event for when the lockdown is about to begin and after it ends?

I have seen the following documented:

• PLAYER_REGEN_DISABLED is sent before protected frames are locked down
• PLAYER_REGEN_ENABLED is sent after protected frames are unlocked

• PLAYER_REGEN_DISABLED : Fired whenever you get aggro, as normal regen rates are disabled during combat.
• PLAYER_REGEN_ENABLED : Fired after ending combat, as regen rates return to normal. Useful for determining when a player has lost aggro.

Via WoW Forums