Well, Willamina, you summed that up rather well. And it looks like everyone here agrees with you.
I'm still not 100% convinced, though. I mean, yes, God is all-knowing and all-seeing. I get that. God knows what I'm going to do with my life because he can see it all evn though I can't. I get that, too, and I'm totally on board with it. But even the Psalms that bopeep1909 quoted above don't stand to me as a clear statement of, "You didn't exist until I created you as a fetus in your mother's womb."
Part of the reason I ask has to do with some personal experiences I've had. There have been people that I have met in my life who somehow seem familiar to me. One of my friends in high school, for example. He's a die-hard atheist; I'm a (slightly confused) believer. He uses Linux; I use Windows. Our interests and personalities are quite different. But I met him once, on a class field trip, and after maybe two minutes of conversation between museum exhibits, we were best friends. There was something inexplicably familiar about him, and to this day, I just can't shake the feeling that I knew him before I met him.
To me, experiences like that and the verses in Jeremiah suggest that, just maybe, we did exist before mortality began. Maybe we were just God's thoughts, and maybe we just happened to bump into each other at some point.
Does anybody think it's even possible, or am I way off track here?