Bruce Perens (co-founder of the Open Source Initiative) has been opining about the difference between open source hardware and open hardware. People have compared the debate to the difference between open source software and free software, and are concerned that it might become as divisive. I’m not so sure. I think they are two different things, and they can co-exist peacefully.
To my mind Open Source Hardware to be hardware which comes with the recipe/blueprints/source code whatever so that you can reproduce it yourself, and Open Hardware to be hardware that comes with complete specifications so that you can interface to it without any nasty surprises and without necessarily knowing what goes on inside.
Open source hardware is better (from the user’s perspective), but open hardware is definitely a step in the right direction.
Open source hardware almost inevitably relies on open hardware: for example, you can have all the specs to a simple integrated circuit like a 555 timer, but you don't need to have the information necessary to build one.
Or for even harder hardware, you may have the specs of a bolt (thread pitch, diameter, length, head type, tensile strength, general resistance to corrosion etc.) but you're unlikely to know the exact makeup of the alloy used to make it, or how it was tempered etc. So most simple electronic components will be open hardware.
The only thing you have to be a bit careful about when talking about open source hardware is that it's more difficult to draw the boundary between source and object than it is with software. For example, I'd say an open source car is still an open source car if the complete design docs are available, even if the motors are individual proprietary units and you don't have the source for them, so long as the specification of the motors is sufficient for you to do what you want with them and they do what they are expected to do with no nasty surprises.
If you take a maximalist approach, and want instructions sufficient to enable you to synthesise a car from a bunch of atoms, then you will spend most of your life sadly disappointed. Luckily, I’ve never met anyone like this in the open source software (sorry, free software) world :)
On an entirely unrelated note, I’ve never met Richard Stallman. It’s a pity I’m on holiday on 25th August, as I hear he’s speaking in Birmingham