Steam Linux Beta and L4D2 game will most likely come to Linux next month.
In a recent post on Left 4 Dead 2 blog, Valve announced that Steam Workshop is coming to Left 4 Dead 2 along with Linux support.
All of this will also be available not just on the PC and Mac, but also for Linux users as well starting in the middle of next month and rolling out from there. We will have more details as we get closer to launch.
Steam Linux beta app, Portal and Serious Sam 3 have already been listed in Steam content database. A Linux platform option has also been added to Steam Greenlight page. So there should be a proper announcement from Valve soon.
Interestingly, Ubuntu 12.10 will be released on October 18th. Something cooking between Canonical and Valve?