There are a few things you can do to make it a little more difficult to swipe your SWF, but in the end you have to remember that if someone wants it, they're going to find a way to get it. If I were you I wouldn't get too paranoid about protecting it.
Three of the easiest things to do are enable "protect from import", domain checks and personal security sequences.
"Protect from import" is probably the easiest, and gets rid of those who don't know what they're doing. It stops your SWF being opened in editing mode in Flash. To enable it, publish the SWF and check the option for "Protect from import" when it pops up.
Domain checks involve gettting the SWF to check the URL under which it is currently running, and compare it to the URL it SHOULD be playing at (i.e. your site). If the two don't match, the movie stops at a frame saying "Come and see this movie at
so that you still get the traffic.
The security sequence was something that a couple of us thought up a while ago ... it won't stop anyone stealing your SWF, but it will prove that you created it, making it a tidy case of suing for breach of copyright. All you need to do is make a movieClip which accepts a sequence of keystrokes to display a message about your ownership. Both the sequence and the ownership notice can be stored in a TXT file which does NOT need to be put on the server. By running the SWF locally with the TXT file, the keystroke sequence can be activated, and the message displayed.
Even this isn't foolproof ... if someone figures out what the seemingly useless MC is for they may recode it to show their own copyright notice, but I'm sure someone else on the boards would have a way of tightening this method up slightly.
