They are adding Linux publishing but according to this post (http://blogs.unity3d.com/2012/07/03/linux-publishing-preview-what-how-and-wherefore-3/#comment-4296

They are adding Linux publishing but according to this post (http://blogs.unity3d.com/2012/07/03/linux-publishing-preview-what-how-and-wherefore-3/#comment-42961 http://blogs.unity3d.com/2012/07/03/linux-publishing-preview...), the editor won't be available for Linux. Still though, this means that Mono is being used with Unity on Linux. Unfortunately not the development part.