After מעריב on כל נדרי night (hey, some people are always thinking דקדוק) someone asked me what determines when זְכֹר with a שוא is used as opposed to זָכוֹר with a קמץ. The catalyst for the question was likely זכר כי עפר אנחנו in אבינו מלכנו. It would seem that there is no actual difference in meaning between the two. One interesting clue that occurred to me was that we always find זָכוֹר followed by the word את, as in זכור את אשר עשה לך עמלק and זכור את יום השבת לקדשו. And it seems that זְכֹר is never followed by את, for example, זכר אל תשכח and זכר ימות עולם. Has anyone seen anything specific on this?