Here's my theory.
I have that yucky taste, too, and I noticed it's really bad right after eating. I read a few days ago that when you eat anything with sugar (refined or not) it creates acid in your mouth for as long as a half hour. I think with us being pregnant we are probably so sensitive to that acidity (with our nausea and all) that it just affects us differently than the non pregnant folks. With us eating frequent small meals we probably don't have much time out of the day without that taste. I know I'm wearing my toothbrush out.
Oh yeah, and like pp said I'm sure the prenatals have something to do with it, too.