I agree to some extent, Nicholas: I also use both talk and speak. I have been reflecting on why and when I use the two forms, and I think it depends on the circumstances as to which I woudl use. If we were conversing on the telephone, I mught well conclude with, "... I'll talk to you later about this." But if I had phoned you and was answered by someone else, I would almost certainly say, "I would like to speak with Nicholas, please."

Why is that, I wonder?