While the Valve Wiki page was created on Ubuntu.com a few weeks ago, it cites a new AMD Catalyst driver is needed for being able to properly support the Source Engine on Linux. This new Catalyst driver will be packaged in Ubuntu 12.04 LTS as fglrx-experimental-9. This new fglrx/Catalyst driver that works really well with the Source Engine games has yet to be released (likely it will be fglrx 9.02/9.03 release streams).
On the NVIDIA side, the latest nvidia-experimental-xxx package is needed for fetching the very latest drivers. For the NVIDIA support of Valve's Linux games, you should be good on the 304.xx or 310.xx series.
Lastly, for the open-source Intel driver, the packages within the ubuntu-x-swat/x-updates PPA are required.
For the back-story to Valve and the OpenGL Linux drivers, read Valve's initial outreach looking for OpenGL developers, the exclusive April news, Valve Picks Up Another All-Star Linux Developer, Valve Finds Value In Open-Source Drivers, and Ubuntu Needs To Improve OpenGL Drivers For Gaming.