Weeks ago, Valve published a hugely important statement, announcing that Steam and Left 4 Dead 2 are coming to Ubuntu 12.04, adding themselves to the already existing Ubuntu's "wave" of popular games enrichment.
After the announcement, obviously, the Valve developers have started working on refining, testing, tweaking, developing, etc, parts of their game stack, bundle of development actions oriented towards Linux (Ubuntu), probably with the end goal to deliver zero-compromise fully usable properly optimized games for Ubuntu 12.04 LTS.