first: +1 for a native Linux release
hawkn wrote:There have been countless threads about this.
1. There isn't enough profit from doing it to make it worth the time.
How do you know without giving it a try?
The install base of Linux deployments is actually much larger than people tend to think, especially in places like Brazil, India, China etc. It's impossible to know for sure, since Linux mostly isn't sold, so it doesn't have a "market share". But just to throw in a number: Ubuntu is aiming for 200 millions installs 4 years from now . That is an ambitious plan, but they are going to be pretty big even if they fall short of that number. And that's just one distro!
You should also consider that gamers on Linux are starving for quality games, so any high quality game that is released for them will surely be popular.
I will also link to this post about how many indie developers are planning to target Linux in upcoming releases . Interestingly more than PS3 and Xbox 360, according to that survey.
hawkn wrote:2. There are too many version of Linux, from what I understand.
That can indeed make it more complicated, but I don't think it's a serious issue. First, with a DEB and an RPM, you got the vast majority of the desktop installs covered. Second, I'd think that you need to cope with different versions of Win (or MacOsX) as well; Third, packaging and dependcy management is generally MUCH saner on Linux. Once you used a package manager on Linux, you can never go back to the stone age that is MSI installers. Fourth, I'm 100% sure that the Linux crowd will be very forthcoming to help with the packaging if Runic would reach out to them. There are some very dedicated chaps in those communities who would certainly put in some voluntary effort to help make the game run on their OS. Those are not all hobby programmers, btw., for example Ubuntu has an app store run by Canonical, and their employees would certainly be interested to help get the game into their store. 
hawkn wrote:It's simple math, really. Which is more appealing? A 5% market share, or closer to 90%?
Of course they will release for Windows, but the idea is that making a Linux port in addition to the Mac port may not be that much extra work and may be worth the effort.
 http://www.techdrivein.com/2011/05/goal ... -in-4.html
 http://www.ubuntu.com/ubuntu/features/u ... are-centre