Some books are now published as pdf's, with no DRM - except for an invisible identifier (watermark), and your name on the bottom of each page. The upshot is: you can copy it as much as you want, with no hassles at all, but everyone will know it was you.

For example: http://www.informit.com/store/faq_nondrm.aspx

I find this very appealing, and it seems to be working (at least, publishers are adopting it).

What do you think about applying this to other downloadable products - particularly, programs?