Is it possible to use WPF on unix?
WPF under Mono does not exist.
At this point, no group in the Mono project has plans to implement Windows Presentation Foundation APIs as part of the project.
The mono team propose using Silverlight/Moonlight instead:
Silverlight implement a subset of the WPF APIs and is available on Windows, MacOS X and through our own open source effort Moonlight it is available on Linux and other Unix systems.
There is a library called Silverform SDK that aims to provide cross-platform WPF and Silverlight implementation.
The library is implemented in managed code and currently works with OpenTK and Unity3D as render backends. Major functionality, such as binding, layout, main controls and primitives, has already been implemented (check Unity web player demos here). Initially it has been focused on Unity3d render, while support for standalone Mono applications will be added as a separate build in the future.
Disclaimer: I am one of the developers of the library.