I already reported about prior attempts. Now one of the Unity developers uses his "do-what-you-want" Fridays to experiment with a visual scripting system.
That 'development experiment' is currently called "Visual Logic Editor". You will notice that it's purely event-driven plus it contains low-level logic-components. Thus it looks more in the style of Crytek's Flow Graph and maybe a bit like Unreal's Kismet.
This kind of system are also used a lot in procedural content generation like found in .werkkzeug, mapzone/substance air and genetica. Or for Shader creation like with ShaderFX or mental mill.
Virtools hierarchical graph works quite differently as data and program flow are modeled separately – Kismet seems a bit like like a hybrid in that area, but as I never tried I don't know for sure. I hope at one point VLE at least goes in the direction of encapsulated hierarchical flows, because otherwise it may become quite messy with growing logical complexity – most systems add that at a later point anyways.
Example of how messy a Flowgraph can look like (found on the web):
The key for visual scripting system are hierarchical ones. It's like a LOD – level-of-detail system or programming functions/methods plus it facilitates re-usability. Personally, I prefer modeling state-machines that defines behaviour with these kind of of hierachical visual-scripting systems, as you can *look* at the relations and patterns. In code it's not that visible at a glance, in my humble opinion – even when using Co-Routines.