As most people are probably aware, the new QuickTime X Player in Snow Leopard has no preferences. One of the preferences I greatly missed was to automatically start playing a movie when it was opened.
Well I'm happy to say you can re-enable that behavior with the following Terminal command:
defaults write com.apple.QuickTimePlayerX MGPlayMovieOnOpen 1
The movie will now start when opened without you having to press the Play button.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090830122703512