Sort of stumbled over this today after taking my laptop out for use and then coming back to the house to use. I hooked the mbp up to the ext monitor, reconnected ext keyboard and mouse, got started working, closed the laptop and the ext monitor went dark. After about 20 minutes of screwing around with settings, and looking for solutions, finally found that the laptop will not mirror an ext monitor unless connected to ext power. What is that all about?? Is this just a supposed battery saving option??