If you want to hide a folder where no one will ever think to look, control-click on any application and select Show Package Contents. Double-click on Contents, and drag and drop the folder you are wanting to hide into the Contents folder. If you are still paranoid someone will find it, open a terminal, navigate to the Contents of the application you stored the folder in, and type:
$ cp -r filename .filenameIf you'd rather just rename, instead of duplicate, the file, do this instead:
$ mv filename .filename[robg adds: I can think of a few downsides to this technique, the biggest of which is that if you upgrade the application to a newer version, you'd probably do so by erasing the old after installing the new. If you forgot about your hidden file, it's gone. I prefer using protected disk images for secure data on my laptop, but this method definitely works -- just make sure you know where you put your file, especially if you choose to make it invisible.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040423010601497