Notice this morning that Jamba (which was the only addOn I had so far installted (cleaning things up)) was causing "Interface action failed because of an AddOn" whenever I entered combat for the first time. So did a trace on this (using taint.log), which took me to ...

JambaCore.lua, line #698

Code:
ClearOverrideBindings( self.keyBindingFrame )
I changed this (after some Googling (seems patch 3.1 is responsible)) to ...

Code:
if not InCombatLockdown() then
	ClearOverrideBindings( self.keyBindingFrame )
end
... and am no longer getting the error message.

(I'm fairly new to AddOn code though so use at own risk!)