I noticed this was brought up in June of last year with no further comment on it, so I thought it was worth reviving the discussion!
Part of this is a process thing (and me being a klutz...) but I often notice after it's already too late that I sent an older approved version rather than a new draft that I forgot to approve.
On Wordpress there is a warning of a newer draft that has been saved as a bold heading. I'd like to see that kind of thing for Marketo! At least some kind of mental jog that I need to see why there is a new draft.
Alternatively, maybe not let an email program run if there is an unapproved draft? That may cause more problems then it is worth, but it seems like you'd only have an unapproved draft if there were still changes in the works so you would either approve the draft or delete it.