echo  "x11-drivers/nvidia-drivers ~amd64" >  /etc/portage/package.keywords

fixed this problem for me when I upgraded to kernel 2.6.27-r5 from 2.6.25-r7
  ----- Original Message ----- 
  From: Peter Bleszynski 
  To: gentoo-amd64@lists.gentoo.org 
  Sent: Saturday, December 13, 2008 2:53 AM
  Subject: Re: [gentoo-amd64] Re: can not install nvidia driver


  I was able to install nvidia-drivers while running gentoo-sources-2.6.27-r4 using the ebuild posted here: http://bugs.gentoo.org/show_bug.cgi?id=246425


  On Fri, Dec 12, 2008 at 11:01 PM, Duncan <1i5t5.duncan@cox.net> wrote:

    Mansour Al Akeel <mansour.alakeel@gmail.com> posted
    49433267.1090205@gmail.com, excerpted below, on  Fri, 12 Dec 2008 23:56:23
    -0400:


    > In file included from
    > /var/tmp/portage/x11-drivers/nvidia-drivers-173.14.09/work/NVIDIA-Linux-
    x86_64-173.14.09-pkg2/usr/src/nv/nv.c:14:
    > /var/tmp/portage/x11-drivers/nvidia-drivers-173.14.09/work/NVIDIA-Linux-
    x86_64-173.14.09-pkg2/usr/src/nv/nv-linux.h:107:27:
    > error: asm/semaphore.h: No such file or directory


    The arch-x86 (including 64-bit) header files were reorganized in kernel
    2.6.27, and as that says, asm/semaphore.h (or more like asm-x86/
    semaphore.h) no longer exists in .27.  I won't install proprietaryware so
    no nvidia for me and no direct experience with it to help you with, but
    it's likely nVidia hasn't updated to take account of that yet (or if so
    it'd probably be with the newest drivers, which may not even be in
    portage yet let alone stable).

    You did mention switching your running kernel back to 2.6.26.x, which
    should work, but did you switch your kernel symlink back to it as well?
    If not, it's probably still pointing at the 2.6.27.x kernel, thus you'd
    still be getting the error.

    --
    Duncan - List replies preferred.   No HTML msgs.
    "Every nonfree program has a lord, a master --
    and if you use the program, he is your master."  Richard Stallman