I feel like allowing a character to use shield bash as an AOO when he was using it as a regular shield (getting the AC bonus) his last turn can be a little unfair. After all, if he takes the AOO right before his turn he can go back to using the shield defensively and not ever see the AC "penalty" of using the shield as a weapon.
I think I'd rule that you could threaten with a shield only if it were already in "weapon mode". Basically, the shield is in weapon mode when you used it to attack and had given up its bonus to AC. If you had Improved Shield Bash (or any other ability/effect that allows you to use the shield as a weapon without losing its AC bonus) then the shield is always in weapon mode.
The same goes for a wizard holding a wand. If he had beat someone over the head with the weapon on his last turn then the wand is in weapon mode and could be used to AOO. Any feats/effects that let him bash with the wand and use it as an actual wand would let him AOO with it whenever he wanted.
