I believe Humble Bundle has a partnership with a consultancy that will take care of porting a game to other platforms to enable their Mac/Linux/Windows value proposition. Valve could probably do something similar (though I imagine it's a lot more work to port a AAA game than an indie platformer).
If you're using an off-the-shelf engine (Unreal engine, say), it's probably somewhat easier, as some of the work would be one-off, at least in theory. But I don't honestly know how that breaks down.
I can't speak for recent Humble Bundles, but the linux experience of earlier ones was inconsistent to say the least. Some games were as if they were intended to be native. Others just didn't work or actually didn't exist. Others worked, but were buggy... and never got the update fixes. Maybe they have become better?
