This is a long-term enhancement idea: when multiple people open the editor on a page at the same time, we should display some notification that this is the case. This could be tricky to do. One idea I had was we could use WebSockets if available to share this information; once in the editor, open a WebSocket connection to "ping" periodically. Indeed, this could even eventually be used to allow some sort of collaborative editing, with people sharing ideas for what needs to be done. Obviously that would only work in more modern browsers, but that should be increasingly well supported before long.
+1 for collaborative editing. WordPress notifies you author when someone else is editing an article, but whenever I see this notice I immediately think "Oh... you mean I can't see his edits as he's making them?"