I Just had this exact problem first time i tried FCP7 on my brand new Feb’11 Macbook pro which is using min dvi to HDMI adaptor to a Benq 24″ 1080p monitor. And seemed to have found a work around, in my instance anyway.
It seems if you start final cut with the monitor attached it will always be dark. I found that if you start final cut and then connect the monitor the viewers are normal. This seems to work no matter what your monitor set up is, ie your using 2 displays, mirroring or external display with mac in clamshell.
Hope that helps someone
Steve