DRM files is bad, on sd card or on simple file.
I don't have a pda with sd (just MS) and I don't think I'm alone on this.
Also imagine in 4-5 years, are you sure that the ebook you have on your sd card would be readable? Are you sure that sd card will exist ?
This is even worst thant actual drm, it attach you to a physical device even more.
Just look at the Baen mean of selling ebooks, do you think there is a lot of piracy on their cheap drm free ebooks?