This shouldn't come as a big surprise since the Unity game engine coming to Linux has already been brought up previously and we've known it was being worked on, but now with Unity 4.0 it's finally materialized.
Aside from the Linux support, this updated 3D engine has Microsoft DirectX 11 support, Mecanim animation system, 3D texture support, major optimizations, refined component-based workflows, geometry data improvements, and much more.
Additional details on Unity 3D 4.0 can be found from the Unity 3D Forums. Embedded below is also a video about version 4.0 of Unity 3D.
Unity 4.0 is currently available for pre-order.