I bought my copy of Agile Rails Development after using a copy purchased by someone else. Their name was at the bottom of each page, and though it was kind of weird seeing it over and over, it didn't seem like such a big deal...I bought my own copy soon after because the book was worth buying.
But now the more I think about it, I'm just encouraging these folks to start pasting my information in my documents, documents that could get pilfered in a laptop theft or accidental folder sharing—there are plenty of stories of P2P app users accidentally sharing their entire HD.
Whenever I see some strange DRM scheme I think about any of the introductions to one of Cory Doctorow's books. The ones where he points out he's given away hundreds of thousands of digital copies AND gone through numerous print editions. I still buy his books because they're easy to read as a book, would cost me a heck of a lot more to print it, and yeah, I want to support him as a writer.
Anyway, Cory tells it better than me, you can read it for yourself (gosh!) on his site.