Wow, I can't even remember.... I'm sure it was in high school. But I very distinctly remember when I was a freshman in college, in 1991, a friend introduced me to this visiting professor who had a roundtable discussion on alternatives in religion -- not part of a class or anything, just people getting together to share ideas -- and he gave a very convincing pitch on deism, pantheism, and pandeism. That was definitely the first time I heard of pandeism, and probably the first time deism had been addressed to me in a really complete way. I'll see if I can find the prof's name, and give him a ring!!
I heard of Panendeism only recently, by comparison -- I think it was only coined recently, tho. Saw it on Wikipedia about a year ago, before the admins deleted the article.