I’m currently using Grome v1.1 from www.quadsoftware.com for some terrain editing and object placement for a military project. I think it could be an excellent companion to Unity 2.0. However, Unity doesn’t support OSG format. The Collada format includes multiple UV sets and layering, but I don’t know that Unity will read anything beyond mesh and 1 UV set.
Does Unity support multiple UV sets in Collada format?
By the way, I notice feelingsoftware has released a new Maya Collada import/export plugin recently (including OSX)