When I click the Insert/Edit Link in the post editor, while inserting the link, there should be an option to nofollow the link if needed.
It could be a simple checkbox for nofollow, or maybe a radio button/drop down choice with dofollow selected by default.
Will be very useful as right now you need to go to text edit and add it manaully everytime you need to. With so many Google penalties, this would seem useful. Should not be difficult to implement also.