I'm also having a problem downloading my models I've uploaded to the 3D Warehouse. They download, but before they finish SketchUp 8 prompts me to fix problems.
Several of my models seem to have this problem, others do not. This one (linked below) in particular also was rated poorly by a user having problems with the Collada file:
http://sketchup.google.com/3dwarehouse/details?mid=f1c2d11ede169b0e9f2b2f789276c61a
The "results of the validity check" after clicking on "Fix Now" has many lines with messages about:
"CEdge (258316) and CEdge (258544) connect the same 2 vertices - fixed" ... etc.
I tried changing the geometry, redoing entire sections, simplifying and even deleting most of the model, re-uploading it and still was getting the same problem. Any ideas? I would really like to fix this and provide quality models for other people to use.