I have made my ET installation work following these steps, please note that some of them may be unnecessesary and it may not work on every system configuration:
1) Get all ia32 packages from synaptic.
2) Install package "linux32" (it's a uname wrapper )
3) Install ia32-libs-more from Subterrific's package: http://illadvised.com/~jason/ia32-extras-0.1.tar.gz
4) Get 32-bit OpenGL DRI libs for your graphics card.
4.1) (ATI cards only) Go to www.ati.com and download the 32-bit rpm of your driver (same version as 64-bit).
4.2) (ATI cards only) Unpack "fglrx_dri.so" (should be under "/./usr/X11R6/lib/modules/dri/" inside the package).
4.3) (ATI and nVidia cards) Put it in "/usr/X11R6/lib32/modules/dri". That directory should not exist, so you have to create it.
5) Run the latest ET setup binary (2.56) by doing "linux32 sh et-linux-2.56.x86.run".
6) Follow the setup instructions and complete the installation process.
7) Put Kareema's lines (the bold ones) in your startup script ([game's install path]/et):
8 ) Fire up the script by doing "linux32 sh et" (it should work without doing "linux32", but I haven't tried it yet).
# Needed to make symlinks/shortcuts work.
# the binaries must run with correct working directory
Step 3 should be optional, but it can't hurt to have a few more 32-bit libs lying around I haven't tried removing them.
Also, setting LD_LIBRARY_PATH should be optional, I haven't tried it yet.
These procedure should also work for Q3, but I haven't tried that...
For those of you running nVidia cards, you could try the following steps (I don't know whether they work or not):
4.1) (nVidia only) Go to www.nvidia.com and download the 32-bit linux driver (get the same version as your current 64-bit driver).
4.2) Only unpack it by doing "$ sh NVIDIA-Linux-x86-1.0-7167-pkg1.run --extract-only"
4.3) Copy both the "libGLcore.so.1.0.7167" file and the "libGL.so.1.0.7167" file from "...[place where you unpacked]/NVIDIA-Linux-x86-1.0-7167-pkg1/usr/lib" to "/usr/X11R6/lib32/modules/dri". Create the directory if needed.
Continue with the remaining steps and post back your results, so we can know if it works!
[EDIT]I've been told this method is not working on nVidia cards, so I guess that makes this Howto for ATI hardware only.