It seems that a bug of Google SketchUp prevents to read all data stored in SKM, and also prevents some things to be written inside a SKM file... more details can be found here
EDIT: changed the link, now accessible - http://twilightrender.com/phpBB3/viewtopic.php?f=11&t=3173&start=10#p24884
It's strange that if SOME of data gets inside that SKM file, it can't be read after that... and if SOME of data can be written, why can't be everything?

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