I was having the same problem as everyone else unable to get a 1280x800 resolution on xorg by just modifying the xorg.conf file. I'm running Slackware 11 RC4 on a Dell Inspiron B130 with the Intel i915 video chipset.

Download 855resolution from : http://perso.orange.fr/apoirier/
If you notice it says it supports 915

Steps I took :
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tar zxvf 855resolution-0.4.tgz (or replace with your filename)
cd 855resolution (get into the 855resolution directory)
make (compile the program)
su (to become root, you'll need write access to several files)
make install (install the program)
855resolution -l (lowercase L, note the mode you don't want to use)
855resolution 5c 1280 800 (5c can be replaced for any res you won't use)
855resolution -l (5c is now replaced with 1280x800)
vim /etc/X11/xorg.conf
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Go to the Screen Section of your xorg.conf file
I modified mine to :
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1280x800"
EndSubSection

*Note : If you were having trouble like I was and added
HorizSync x - x
VertRefresh x - x
Modeline "1280x800" x x x x x x x
to your monitor section, Comment it all out. Mine works perfectly
without it
====
exit (get off of the root account, if you are not root, don't exit)
startx
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This worked on a full install of slackware 11 rc4 using the default vmlinuz kernel.

Btw, if this works for you as well on a reboot it will revert back to it's ugly nonfunctional resolution. As a workaround I did the following

echo 855resolution 5c 1280 800 >> /etc/rc.d/rc.local

On reboot you should see
** Patch mode 5c to resolution 1280x800 complete