If you're reading this blog for a while, attended one of my recent presentations or are dealing with SD Card support in GPL licensed Free Software, then you probably know about the never-ending issues around that subject.
For considerable time, the SD Card Association appeared to have regulations in place that basically prevented any SD card driver to be GPL licensed, because of NDA's and other stuff. I can only imagine that this was because of their DRM scheme [which I've never seen anyone using, btw]. Instead of creating separate standards, one openly documented for the basics, and one for the crypto function, the NDA covered all of it.
Nonetheless, various projects have meanwhile developed their own GPL licensed SD card support, since it is reasonably close to MMC (which is publicly documented), and the many proprietary drivers that can be re-engineered.
Very recently (it seems April 03, 2006), the SD Card Association has published a set of "Simplified" standards. You can get the documents from their homepage. There's one for SD Card Physical Layer, one for the SD Card Host Controller, and two related to SDIO. Obviously this publication is a couple of years too late. But let's try to be positive and look forward and not to the past.
So it seems their policy seems to change at least a bit. A very welcome move, although for my taste there is still one document missing: One that describes the actual commands on top of the physical layer.