D20 Modern doesn't have alignment.
Planescape does.
What do we do about it? 
Just bring it back? I don't see any reason why this wouldn't work, but plenty of reasons why leaving it out could cause problems.
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I'm running a Planescape campaign with GURPS rules based on Omar's conversion linked below and no alignment. I think you don't need alignment as a game-mechanic to explain the Great Ring and an ethic-moral system, so just forget about alignment would be my advice.
http://www.sonic.net/~rknop/Omar/planescape/gurps/characters.html#alignment