Comment 33 for bug 12637

I don't know, so far the debugging hints in here haven't been too detailed/helpful to actually be able to analyze/resolve the problem as an enduser.

I think that since it happens immediately after bootloader and when loading kernel, this may point to vga mode configuration at kernel bootup
triggering the dimming issue (probably some vga BIOS interrupt call either mis-called by Linux or buggy implementation which happens to dim
the display, too).
Please add the vga=ask kernel boot parameter / LILO parameter and try to find out whether choosing some particular value does NOT dim
the display; vga=normal or vga=extended may also be helpful.

If this simple enduser testing doesn't find a way to avoid the dimming, then it might help to completely disable ANY vga card fiddling in kernel;
this might be achieved by trying to completely disable the linux/arch/i386/boot/video.c/set_video() function (place a return; statement near the
very top of this function).
Further places to investigate at would be all video card related files in linux/arch/i386/boot/ (e.g. video-bios.c, video-vesa.c, video-vga.c and video.c).
Even an enduser could add all sorts of disabling "return;" statements to various video functions there to find a way to prevent this from happening.
And yes, these things obviously mean compiling a kernel from source (take /boot/config... file from existing kernel and copy to linux-2.6.XX/.config, make oldconfig,
make-kpkg kernel_image and install it, tell bootloader to use it, that's one good way to do it).

HTH and good luck!