I came across a tidbit of news this morning. If you are setting your SL Viewer to any language other than English, the Collada import for mesh may not work.
You’ll see the problem in the import preview window. You’ll see polygons with normals reversed and vertices here and there out of place. If you try more than one import, you’ll see the reversed normal polygons and the out of place vertices change and which are messed up are unpredictable.
For now the problem is corrected by switching the viewer to use English.
If you are seeing the problem and can reproduce it on demand file a JIRA bug report with all the steps.
The discussion of the problem is in the SL Forum: Unpredictable mesh import.