Oddly enough, the g-mail default is to top post. How 'bout that?
The only alternative is putting the cursor at the bottom of the email to start with, which would thoroughly confuse people. It would also encourage more overquoting - at least when you top post and overquote at the same time, it's easy enough to ignore. Having to scroll through an entire conversation to reach the message is very annoying (especially if someone is replying to a digest email...).
Not top-posting has to be a concious decision, you can't enforce it by default cursor positioning.