A new Steam for Linux troubleshooting and support page has been launched. This page contains a number of guides on installing correct graphics drivers, updating repositories etc.
Currently, Steam for Linux supports Ubuntu 12.04 LTS with the Unity desktop only. While Steam should run on any desktop environment you prefer in Ubuntu, it is likely that official support may not be provided if you are using any other desktop shell. Additional distributions will be examined for support in future.
A Ubuntu wiki page was also launched few days back with some tips on updating graphic drivers, as required by Steam.