Shoutbox

What you're requesting isn't invading anyones privacy at all as you only want to get notified when someone starts typing a message to you, i.e. a popup saying "user xy has started typing a message" and don't want to know what he's typing in advance.

See CookieRevised's reply to [request]Key press notifier for a script that does exactly that.
(note: it doesn't display a toast telling you that the user is typing when you already have a conversation window open with this contact)