While I think it's fine for someone to post an application (esp. a free one) that takes advantage of this hidden QuickTime feature, I think everyone should know how easy this is to do yourself.
I found this on the internet (possibly even here in a comment at MacUpdate). Just create this AppleScript:
tell application "QuickTime Player"
I've bound that AppleScript to the F5 key when QuickTime Player is the active app (using Keyboard Maestro). Then, I can activate full-screen whenever I want, right from QuickTime, and don't have to remember to open the movie first in a separate app like this.