I would think that steam will see that Knytt Underground is on the PS3 store, and quickly add it to their own. If Nifflas/fastspring/whoever published or does the communication with steam makes it clear that there is a linux version in beta, there is a good chance that it will be added to the linux catalog.
At the moment, steam is only "supported" (ofc it can be made to run with differing levels of success on other distros) on ubuntu. As such, if steam adds it to the game library, many of the casual linux users of ubuntu, who would also have steam installed, would be attracted by it.
Has anyone tried using wine to run it? Occasionally wine can almost perfectly emulate programs (see trackmania, ableton live etc).
Ps. I played through the demo on my winpc, and it is the most ambient and interesting game I have played since WADF. The atmosphere is spot on, Nifflas, you have created a masterpiece
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Edit:
I installed Mint 14, and it runs flawlessly. When in fullscreen, it runs at ~30 fps, but this is due to the bad integrated graphics on my linux computer. When in windowed, it runs at fullspeed (60 fps). Once again, great work Nifflas, the game plays and looks amazing!