iWork' 09 introduced a new file format for Pages, Numbers and Keynote documents. Previous versions of iWork documents were kept as packages, allowing you to peek at their contents with the Show Package Contents contextual menu, but this trick does not work any more by default in iWork '09 -- the new format appears to just be a flat file. (You can set the preferences within the iWork '09 applications to save files as packages, if you prefer.)
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.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090225034801527