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I had no idea that a Mini or a Mac Pro cared if you ran it headless.
Personally, I can think of times when I would have probably been better off without my head...
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I would hope/assume that if you were running OS X SERVER that this wouldn't be an issue?
I've never had any experience running any computer without a monitor. "Server" operation may actually be the real market for these devices. I noted in the Amazon comments that many were from Windows users who probably have more headless hardware because of it is often much cheaper.
I think it is more a function of Apple using more software control of the video. I suspect that most non-Apple monitors have built-in controls that allows them to work with practically any hardware. Also, "Servers" probably don't have much use for the Menu Bar and have the Server access/controls window open all the time. Being 'headless' also negates the old phosphor burn in and even the LCD ghosting problem. I'll probably order one today... using my iMac and screen!