I see a few things to think about here:
Will "Steam for Linux" be something that you can just apt-get into your free as in freedom Distro and start running AAA games, or will it in fact be something highly proprietary which is designed for a specific subset of devices which happen to run Linux kernels?
I see a lot of people here discussing "market share" but as has been proved by Apple market share is certainly not everything. In reality the computer market is broken into a number of distinct markets that have overlap and some of these are more lucrative than others.
Pricing structure will make a difference here, I wonder if the Linux games will be the same price as their Windows counterparts? I remember a few years ago there were a number of companies that tried to break into Linux gaming but the prices for the games were astronomical. Many 5 year old games that could have had for Windows for about $10 were selling for $40+ on Linux.
Linux Steam could also focus on other styles of games too, perhaps there is more demand amongst Linux users for highly cerebral RTS & RPG games rather than the latest COD that could be sold at a higher price? All you would really need is one or two "killer" titles that were Linux exclusive (think XBoX and Halo) to make whole categories of gamers take it seriously. Of course it is both a blessing and a curse in the sense that stuff developed for Linux is often easier to port to other platforms than the other way around.
As a Desktop Linux user myself I am not too concerned with "market share" , to an extent I couldn't care less if I am the only the person that uses it. What I really care about is being able to use Linux for all of my day to day tasks (I have a Windows dual boot but I would rather skip the step of having to power cycle my computer at least once a day).