How-to force iMac to output only to external monitor?

As far as I know, there's never been a way to do this... the best I can get to is to make the internal display have a black background and make the external display primary -- but this is a hassle, and it's obviously not "off."

 

I second that. Mirror screens.

Dim main screen to darkness and that is the best you can do without a potential software hack. There is no setting that is user accessible for this. There MAY be a hack for this, but I am unaware of one offhand. IME, hacks of this type do lead to potential instability.

I have my iMac hooked up to anice Cinema display and th iMac is tucked away out of sight with the same dimmed configuration.

I needed the features of the iMac without the cost of the MacPro with the screen real estate of the Cinema. The mini was too underpowered. So I am that mythical mid tower Mac users without a product.