If you want to use Flash then you can secure your media streams using a combination of the RTMPE protocol (it's the 'E' part that's important) and SWF verification. This isn't unbreakable as the data has to be decrypted to play and isn't protected with DRM, but it provides a reasonable deterrent, and is probably sufficient to stop the majority of people trying to use your data.
More security is available with DRM protected media. Adobe have a version that works with Flex but I don't believe it's available for Flash at the moment. Other than that you've got Apple's or Microsoft's implementations, both of which are pretty secure (note that Microsoft have just released their next-generation PlayReady Server SDK for DRM applications).
Determined crackers will get your data no matter what if it's that valuable to them; all you can do is put up enough roadblocks to make it more hassle than it's worth.