I am using Chromium on Ubuntu, installed from the PPAs on Launchpad.
Here are the steps I used to build O3D on my 64-bit Linux:
sudo apt-get install devscripts libglu1-mesa-dev cdbs libgtk2.0-dev libgconf2-dev
tar -xvf depot_tools.tar.gz
export PATH="$HOME/depot_tools:$PATH"
mkdir o3d_chromium_src; cd o3d_chromium_src
gclient config http://src.chromium.org/svn/trunk/src/o3d
gclient sync
export GYP_DEFINES="target_arch=x64"
gclient runhooks --force
cd o3d/build/
hammer -f o3d_main.scons
cd sconsbuild/Debug/
sudo cp libnpo3dautoplugin.so /usr/lib/chromium-browser/plugins/
sudo cp lib* /usr/lib/chromium-browser/
Played Infinite Journey at http://blog.largeanimal.com/demo/
It was cute, looked great, and showed that 3d games in the browser are coming.
No luck with 3d acceleration though, all the games run quite slow, but it looks interesting.