I like Diablo. And from the short trailers and gameplay videos, I got even more hooked. And I caught myself thinking "I am so buying the new Diablo 3". But I use Linux exclusively. And I don't think Blizzard will go the effort to make a Linux version(It took them like only 10 years for a Windows version :P)
But is it actually possible? If a wacky group of Blizzard programmers took the finished game source code(Diablo 3), could they make a Linux version of it? Given the required resources and time and everything else?
And most importantly, will it be worth the effort? Is Linux known as a solid platform for proprietary software?
Making a game available on multiple platforms takes only as much effort as the developers have designed it to take. Porting an engine designed to be portable is practically effortless; porting one that isn't can take as much time as rewriting it from scratch.
Note, however, that the engine is only a small part of the development process. A lot more time is spent on the game assets, including maps, textures, sound, scripts; balancing gameplay; and so.
Still, it's usually hard to justify rewriting that much code if it wasn't designed with that purpose in mind.
Is Linux known as a solid platform for proprietary software?
There's a lot of proprietary commercial software that targets Linux. More than any of the BSDs, which would intuitively be thought of as more proprietary-friendly.