Dell Inspiron 6000, 915GM video, 1680x1050, 2.6.26 kernel, xorg-server-1.4.2
Gentoo's xf86-video-i810-2.4.0-r1 driver (i.e. the intel-2.4.0 driver with some patches selected from git, see http://sources.gentoo.org/viewcvs.py/gentoo-x86/x11-drivers/xf86-video-i810/files/2.4.0/) worked correctly, as did prior driver versions.
Upgrading to xf86-video-i810-2.4.1 (i.e. the intel-2.4.1 driver) resulted in output with a noticeably low color depth. I drew a simple gradient in Inkscape and took some photos - with the 2.4.0 driver, the gradient is smooth, but with the 2.4.1 driver, it's obviously striped.
Weirdly enough, the output of xdpyinfo is the same in both 2.4.0 and 2.4.1
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photo with 2.4.0 driver (smooth gradient)
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photo with 2.4.1 driver (striped gradient)
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Xorg log with 2.4.0 driver
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Xorg log with 2.4.1 driver
Current 2.4-branch has a lvds dither fix, could you try it?
Yes, the dither fix in http://cgit.freedesktop.org/xorg/driver/xf86-video-intel/diff/?h=xf86-video-intel-2.4-branch&id=d5fd845aab4074ed6d0950551e007ea43fd2d0df fixes my color depth problem.
(By the way, as an aside.
Is there any way to get a raw text diff from freedesktop's cgit web interface? Because the diffs that cgit displays are in the form of an HTML table - you can't even copy/paste them into a text editor without destroying whitespace. I had to write a greasemonkey script just to turn cgit's output into something I could actually copy into vim.)
You should install git on your machine ;)
Mark as closed, will bump another version for regression fixes later.