Switching dual dimension OFF in document properties turns dual
dimensions ON - file specific.
This SPR is still open, according to the KB.
My guess is that it's just a SolidWorks quirk. In 2006, I submitted
a SPR for a similar problem in that the checkbox for "Override
Value" was reversed as well. That's supposed to be fixed in 2008
SP0, but we haven't switched to it yet.