reiteration
But, After more thought:
I still agree with the general idea of my previous ruling, but now want to change some small deails to make it better for pcs:
a) The discharge of the energy (of a 'held charge') cannot happen accidentally, but must be a puposeful discharge by the caster/wand-wielder,
b) Loss of consciousness of the caster (including sleep) erases any charges that are being held, (this ruling negates most of the advantage sorcerers have over wizards in this regard. <-- I meant something by this sentence when I wrote it, but darned if I know what in the world it was now!) and
c) If a held charge is not used or otherwise lost before the caster next casts a spell or prepares spells for the day, then the held charge is lost as soon as the next casting begins or the first new spell is memorized.
These three rulings also apply to a held charge from a wand or other device. (See the Whirtlestaffs Wizards Academy midnight encounter with the trolls at the beginning of the game for an example of what I mean by this.)