Gotta agree with you, Lady Pi Phi... that's one of my pet peeves.
My reasoning goes like this: if I call, leave a message or e-mail someone I expect the common courtesy of someone at least returning the compliment. It doesn't have to be immediately, but at least within a reasonable amount of time.
Mind you... I'm pulling my hair out calling and leaving messages for nearly 600 people in my high school's graduating class for our upcoming 20-year reunion. More than enough reason to not volunteer again for the 25th reunion committee in 2008!