It seems Apple did not go very far to create their new format -- the new file format is a standard zip file with a .pages, .numbers or .keynote extension instead of the standard .zip file extension. Therefore, if you replace the file extension with .zip, the file becomes a basic zip file that you can double-click to decompress and access its contents.
[robg adds: This hint originally referred to the file as a .tar file; as the comment notes, that's incorrect. I've updated it to reflect the .zip status, and I removed a warning about the file not working if you recompressed it. On my system, it does work if you re-zip it and rename it to .numbers. However, it then also appears as a navigable package bundle in the Finder...strange, but not file-damaging.]

