Well, this issue has been discussed again some time... The problem is that the USB port can supply pretty low current to power a full midibox, lcd, backlight etc. (500mA I think... correct me someone if I'm wrong). So, a kind of minimal midibox could be usb powered, as long as it consumes less than 500 mA. (you wouldn't want a blown motherboard especially on your laptop...) Maybe for the laptop you could take power staight from the battery... Uclaros.