One area that's heavily under NDA (though perhaps not quite the situation described) is graphics drivers. NVidia and AMD both have high-performance graphics drivers, but the open-source alternatives (reverse-engineered for NVidia, but AMD sponsors their open-source Linux driver) are far behind. They're reluctant to even share hardware specs, let alone the code from their proprietary drivers.

Part of the reluctance is that they don't want to compromise the DRM systems that their products are complicit in, but most of it is patent related. They already have to pay royalties to many graphics has-beens for things like S3TC, and the fear is that if the details of what their hardware and software is doing were publicly available, they'd be painting targets all over themselves. The people AMD employs to help with their open-source Linux drivers are constantly citing "legal review" as the hold-up for releasing new specs or code, but never that they're concerned about making it easier for NVidia to reverse-engineer their stuff.