"It's about time a major player got behind Linux [...]"

What about id Software?

They have a long-standing history of support for the platform, including John Carmack's philosophy and advocation of OSS that has driven his contribution of id Software game engine source code to the community, up to and including id Tech 4.

He even cares so deeply about the topic that he worked to keep id Tech 5 free from the shackles of proprietary code, because he wants to eventually release it as open source. He said (italic emphasis is mine):

"'Do we want to integrate some other vendor's solution, some proprietary code into this?' And the answer's usually no, because eventually id Tech 5 is going to be open source also. This is still the law of the land at id, that the policy is that we're not going to integrate stuff that's going to make it impossible for us to do an eventual open source release. We can argue the exact pros and cons from a pure business standpoint on it, and I can at least make some, perhaps somewhat, contrived cases that I think it's good for the business, but as a personal conviction it's still pretty important to me and I'm standing by that." [1]

I think the reality is that id Software has attempted to build the market-space, but the base just hasn't been there. For example, John had this to say recently at QuakeCon 2012:

"Other interesting sort of PC-ish platforms, we have... the Mac still remains a viable platform for us. The Mac has never required any charity from id, all of those ports have carried their own weight there; they've been viable business platforms.

[...]

Linux is an issue that's taken a lot more currency with Valve announcing Steam for Linux, and that does change, factor, you know, changes things a bit, but we've made two forays into the Linux commercial market, most recently with Quake Live client, and, you know, that platform just hasn't carried its weight compared to the Mac on there. It's great that people are enthusiastic about it, but there's just not nearly as many people that are interested in paying for a game on the platform, and that just seems to be the reality. Valve will probably pull a bunch more people there. I know absolutely nothing about any Valve plans for console, Steam-box stuff on there; I can speculate without violating anything.

[...]

So, it's enticing, the thought there that you might have a well-supported, completely open platform that you could deliver content through the Steam ecosystem there. It's a tough sell on there, but Valve gets huge kudos for having the vision for what they did with Steam, sticking through all of it. It's funny talking about Doom 3, where we can remember back in the days when they're like, 'Well, should you ship Doom 3 on Steam, go out there, make a splash?' ... I'm like, 'You're kidding, right?' That made no sense at all at that time, but you know Valve stuck with it and they're in a really enviable position from all of that now.

It still seems, probably crazy to me that they would be doing anything like that, you know, but, it's something that's not technically impossible, but would be really difficult from a market, sort of ecosystems standpoint." [2]

[1] http://www.linuxgames.com/archives/9374

[2] https://www.youtube.com/watch?v=wt-iVFxgFWk#t=44m28s