Interesting. I have seen several laptops with this issue, but I've never gotten to the bottom of it. I'd be interested to hear what Lemerise has to say about this, since he is well versed with screen issues. A question for you -- when the laptop goes into sleep mode and then wakes up, does the backlight come back on? Most of the ones I've seen like this have a problem where the backlight does not come back after waking up from sleep mode. This leads me to believe it may be a reed switch issue of some type.
Another thought -- there is something called "verbose mode" which displays UNIX-like boot data when a laptop is powering on, and then it looks fine once the laptop has finished booting. This UNIX data displays instead of the white screen with the Apple, which is meant to mask all of this. You can turn verbose mode on and off from the open firmware prompt. Dunno if it's true, but I've been told that some new boards come with verbose mode on by default, and Apple techs sometimes forget to turn it off. Anyway, point is, the fact that laptops with the condition you describe "turn on" when the OS is loaded has always made me wonder if it's something similar to verbose mode, because in both instances the laptop is in a weird state that they "snap out of" when the OS is loaded. I've tried disabling verbose mode on machines with your condition to no effect.
You can also try swapping with a known-good screen to see if that helps, or swap the whole screen hinge with cabling as a next step. That would replace the reed switch and possibly rule that out. I agree with Brady, i.e. a screen is a screen is a screen, and screens are not matched with processor speeds (although I have run into 1.33GHZ 12" laptops which require a specific brand/model screen and have a black screen and fan on high if something other than this brand/model is present...but that is for another discussion).
Anyway, very interested to hear the solution to this one.