What many people might not consider is that a SketchUp Ruby plugin could be malicious. The plugins has full access to the file system and with the Win32 API it can access the registry.
When installing SketchUp plugins one should be just as careful as when you download a application, batch file or system scripting file. Make sure you trust the source and scan the plugin with a virus application before using it.
At the time of writing these words of warning there are no known malicious plugins as far as I am aware of. But one should nonetheless beware.