Freedom to Tinker have a blog post about how HDCP copy protection works and how it might be possible to break it. HDCP copy protection is the next-generation copy protection used in high definition media setups (for example, HD DVD and Bluray discs to HDTVs) which will prevent the media content being copied through the cable using a device.
It is a bit complex, but in this article it has been distilled down quite well, so if you're fairly technically competent and have reasonable maths skills, you might be able to get the gist of it.
The remaining question is, how long will HDCP last before a mainstream crack becomes available? The Content Scrambling System pioneered on DVDs promised a lot, but it wasn't long before people just copied the protection codes from a legitimate DVD player and created decryption software.