If I turn down the brightness to lowest possible, it's still on.
Not like a MacBook, which shuts the display off
I'm assuming this design limitation is because apple is assuming the iMac's screen is large enough to warrant it as a first-use screen. Obviously, someone with a macbook is more than likely going to want to use an external over the 13" screen when given the option, but how many externals are bigger than the iMac's screen? this is most likely apple's line of thinking...