Lucy, I agree wholeheartedly with your stance on Latin singular/plural usage; BUT (there's always a but, isn't there?) data seems to present a singular problem (no pun intended). As was mentioned, data usually aren't counted; in the software world, data almost always come and go in multiples, and we talk about handling single data "elements", so that 'datum' has fallen into desuetude. To further complicate matters, datum seemingly has another (mathematical?) sense of which the plural is datums!