Mike Godwin @ PK has released his "Everything You've Always Wanted to Know About DRM But Were Afraid To Ask" 40-page primer. A free PDF is available here.

I haven't read it yet, but Mike's a smart lawyer and knows his tech stuff, so I expect this will be a good read. A quick skim of the contents shows that it covers both encryption and watermarking systems as well as DRM's application to P2P nets. Mike concludes with his own vision of a "humane" DRM world with minimal government regulation and negligible effects on public domain materials. I'll post more after I finish my first detailed pass. If you've already plugged through it, feel free to post your thoughts.