If for some reason you don't want to close the lid (or if the lid is missing) you can force clamshell mode by setting a small magnet in the area of the trackpad. (That's how the system knows the lid is closed. There's one stuck to the back of the LCD panel.) It doesn't have the be a particularly strong magnet... as the hard drive is right under that area don't be using one of those finger-crushing rare earth ones. ;^b

As I mentioned, in my experimentation it seemed like you could even power on the system in clamshell mode, as long as you have a keyboard and mouse attached.