Existing users, 
AFAIK, when the screen is black on an LCD display, the pixels are all ON, not off... thus the best screensaver, literally, for an LCD is a pure white screen, which turns off all the LCDs, and allows the flourescent tube to shine thru...
That said, if you really want to "save" the live of your monitor, turn it off, since the flourescent tube will gradually lose brightness over its life, and the only way to fix that is to power it down.
BlackenedPixels
black screensaver turns off display without sleeping graphics card
Version: 1.003
black is ON, not off...
Feedback Type: Commentary
Contributed by: tracy valleau--2008 Friday, February 16 2007 @ 08:40 AM PST
Product Platform: MacOSX
Used Product For: Have Not Tried
Recommend Product: NO
Comments
black PIXEL is ON, not off... - patchman_dotmac
What tvalleau wanted to say is that you need to paint the liquid crystal PIXELs white to actually turn them off. For CRTs black is off, but for LCDs white is off. Adding an option for CRT/LCD might be great.Friday, February 16 2007 @ 07:25 PM PST
black PIXEL is ON, not off... - mr.black
Look for white screen, zero brightness in an uncoming release.Friday, February 16 2007 @ 11:43 PM PST
black is ON, not off... - mr.black
Exactly, which is why we programmatically turn the brightness of the attached displays down to zero. On many Mac models this results in the screen LCD being turned off. On other models this results in the overall energy output being reduced.Check it out. Works great on PowerBooks and good on other Mac models. Your mileage will vary.
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Friday, February 16 2007 @ 08:45 AM PST