I have worked a bit in CryEngine and there was a very handy tool for detailing the terrain where by you created a decal from a picture of some detail and just placed it onto the terrain. Is there such a way of detailing terrain in Unity
The guys at Last Bastion Games have a decal system for sale. I've not used it myself but it's an option... http://lastbastiongames.com/middleware/decal/index.shtml
Sadly, no. And to build it yourself, with shadow support, is quite tricky. There is a feature request for it I believe, hope Unity manages to implement it in next release.