First, Install the Nvidia or ATI driver:
Here is Ubuntu's instructions - http://wiki.ubuntulinux.org/BinaryDriverHowto
More info on installing the ATI driver here
Second, download and install the game:
I went to fileshack.com personally but there is a lot of places.
To install go to Applications->System Tools->Root Terminal
Navigate to wherever you downloaded the file.
Simply run the file (./et-linux-2.56-2.x86.run)
and it should bring up a GUI to help you through the process. I pretty much kept everything as default (by default PB is selected, keep this intact since you can't join a lot of servers without it).
Simply type "et" from the command line to launch. Also the games section on your Applications Menu should have Enemy Territory as well (I had to log out for it to take effect)
I found that sound did not work for my setup, giving me the following error:
/dev/dsp: Input/output error Could not mmap /dev/dsp
To resolve this issue I simply typed from root terminal the following:
echo "et.x86 0 0 direct" > /proc/asound/card0/pcm0p/oss
For more info on this click here
Use this command:
chmod +x et-linux-2.56-2.x86.run
su -c "./et-linux-2.56-2.x86.run"
This is not by any means comprehensive. It would be nice to have common commands that are useful as well as more troubleshooting tips (since I didn't have any other problems I can't really help here) since I'm sure there exists a lot more common issues. Please post anything here and I'll try to make the necessary changes to this post so the most up-to-date and correct info is on the first post.
P.S. - With my 9800 Pro I was able to play this game perfectly so even with the inferior ATI drivers so just about anyone should be able to enjoy this game. Have Fun!