SketchUp does not need to be running to extract the thumbnail, only the .skp file is needed.
This means what? If you host a model site, you don't need to manually load every model to generate a thumbnail - it can be automated. It can be used to create an index to your model files, or create web pages using the images. It can be used to create on-the-fly image galleries - and stuff like that.
The extracted png's are 128 pixels wide and about 70 pixels high, although the height varies for some reason.
It's command-line only right now, which is good enough for me.