Another thing to consider about Mac gaming: since iMacs and MacBooks are essentially un-upgradable and they're not particularly designed for gaming in mind, they tend to age faster than a similarly-priced self-built tower PC (at least for gaming.)
I don't think that's going to be an issue for Linux enthusiasts, but it will be an issue for this "magic grapefruit" they're talking about.
I seem to recall gabeN posting to the effect that the average Mac running Steam was better-equipped than the average PC running Steam. If that's true, the non-upgradability might not have entered into it.
As to this "piston" thing here - I don't think upgradability will be an anchor. The whole project isn't remotely feasible unless a distro for build-your-own boxes is also launched. In which case the specifics of this box are more a blueprint for builders and a performance baseline for developers.
Sales probably wouldn't even track net adoption until the enthusiasts build out their custom rigs and start convincing their less-technical friends they can have almost all the same awesome without getting any thermal paste on their hands.
I don't think that's going to be an issue for Linux enthusiasts, but it will be an issue for this "magic grapefruit" they're talking about.