This is more of a warning than a hint. It seems that like OS X applications, Keynote presentations are actually folders of files. I realized this when a colleague sent a presentation to me and I viewed the attachment with a Windows e-mail program. What appeared in the attachment section of the message was all the individual files (pictures, etc.) from the presentation.
So I checked the file in the Finder. Sure enough, if you control- or right-click on a presentation, you get a "Show Package Contents" option. If you routinely use the command-line to move files or backup your files to a Windows or UNIX machine, you should be aware that OS X's UNIX layer and these other OS's will treat the keynote file as a directory. Could cause problems if you don't realize it. You've been warned...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030121062846807