Bug tracking for the Steam for Linux beta client has been moved to GitHub. The Steam for Linux beta now has a public repository and an issue tracking system for creating or browsing bugs related to Steam for Linux. You will need an existing GitHub account[github.com] to create a new bug[github.com].
For more information on creating a new bug, see https://github.com/ValveSoftware/steam-for-linux/blob/master/README.md, located in the public repository.