The GNU Website has statement from Richard Stallman where the founder of GNU and the Free Software Foundation describes his concerns about Valve's plans to bring Steam to Linux (thanks Ars Technica). He explains the freedom to install Steam games will impinge on a user's freedom:

A well known company, Valve, that distributes nonfree computer games with Digital Restrictions Management, recently announced it would distribute these games for GNU/Linux. What good and bad effects can this have?

I suppose that availability of popular nonfree programs on GNU/Linux can boost adoption of the system. However, our goal goes beyond making this system a “success”; its purpose is to bring freedom to the users. Thus, the question is how this development affects users' freedom.

Nonfree game programs (like other nonfree programs) are unethical because they deny freedom to their users. (Game art is a different issue, because it isn't software.) If you want freedom, one requisite for it is not having nonfree programs on your computer. That much is clear.

However, if you're going to use these games, you're better off using them on GNU/Linux rather than on Microsoft Windows. At least you avoid the harm to your freedom that Windows would do.

Thus, in direct practical terms, this development can do both harm and good. It might encourage GNU/Linux users to install these games, and it might encourage users of the games to replace Windows with GNU/Linux. My guess is that the direct good effect will be bigger than the direct harm. But there is also an indirect effect: what does the use of these games teach people in our community?

Any GNU/Linux distro that comes with software to offer these games will teach users that the point is not freedom. Nonfree software in GNU/Linux distros already works against the goal of freedom. Adding these games to a distro would augment that effect.

If you want to promote freedom, please take care not to talk about the availability of these games on GNU/Linux as support for our cause. Instead you could tell people about the Liberated Pixel Cup free game contest, the Free Game Dev Forum, and the LibrePlanet Gaming Collective's free gaming night.