Issues with Beta 2 - 4.0.1693 (technical info that'll probably bore non-GE developers).
The <up_axis> element in the <asset> element of the COLLADA docs is ignored. GE always assumes Z_UP.
Several COLLADA objects that I have created that validate against the 1.4.0 schema, and load fine into other apps, have caused GE to crash, or lock up. I'll try to save one next time I run into this.
The following link says that most of COLLADA is supported. However, I've run into several undocumented implementation issues, such as being limited to triangles with a 16 bit count . Can you post known limits somewhere, or better yet, just switch to 32 bit counts?
http://earth.google.com/kml/kml_tags_21.html#model