If bits of launcher script, photographic evidence and a short video weren't enough to convince you Valve was porting Steam to Linux, maybe you'll take it directly from the horse's mouth: Steam is coming to Ubuntu 12.04. Sick, perhaps, of denying the project, Valve has created a Linux blog to document its efforts -- outlining its history with the open-source OS and making its goals public. The plan is simple: get Steam ported to Linux with full functionality, optimize Left 4 Dead to run at a respectable clip with OpenGL and port additional Valve titles. The team is also asking for feedback, and hope to leverage the ideas of the community.
Although I haven't used Linux in years, this is great news