For the announcement expected tomorrow, it's not Valve/Steam/Source on Linux related, but it's going to be interesting nevertheless for Linux enthusiasts. I just happened to be out in Bellevue when another interesting Linux hardware project was being prepared by another firm. But if you missed those happenings last week, see Valve's Gabe Newell Talks Linux Steam Client, Source Engine, A Video Of The Source Engine On Ubuntu Linux, and A Special Linux Delivery At Valve Software.
The discussions happening within the forums are also worth looking over: 56+ comments, 33+ comments, and 188+ comments.
Anyhow, embedded below is a quick video preview for tomorrow's announcement... (And for those complaining in the forums about the quick and shaky video last week of Left 4 Dead 2 on Ubuntu, this video below was recorded with the same HD camera equipment and tripod I had in my bag while at Valve... It's with intention that I just hastily recorded the L4D2 Linux demo at Valve from an iPhone 4S.)