I'm helping the OSE choose a license for the designs that we produce.
A couple of days ago, I blithely recommended that Marcin license the Liberator compressed earth brick press under the GPL.
Licensing is a largely solved problem in OSS, but on further reading, I found that open hardware licensing is a mess. I'm not a Lawyer (thank goodness :-), but here's by best attempt to understand the situation.
The crucial difference appears to be that physical items are not covered by copyright, which only concerns itself with informational goods.
It's easy enough to do public domain, MIT, or BSD style openness with hardware. These licenses are so permissive that they don't change their behaviour in the new context. They don't need to use copyright to make their provisions enforceable.
The problem comes with the GPL copyleft provisions: Without copyright, enforcing openness of derived works is hard, and I'm not satisfied that any existing license does it well, if at all.
The Open Hardware License (as used by TAPR) was written by a lawyer, but was criticised by Eric Raymond, and I have some misgivings about it too.
There are a substantial number of open hardware projects that are using the GPL as is, and it doesn't seem to be doing them any harm. These include Arduino and RepRap.
I like the obligation to open derived works included in the GPL. I feel that it is politically consistent with the OSE values, and it acts against commercially exploitative freeloading. If it were easy, I would like to enforce that same obligation on anyone selling modified Liberators. I now think that this enforcement may not be possible through copyright.
I also think that it doesn't actually matter very much. We'd like to get the benefit of all modifications to our open designs, but we don't need it. So long as good complete designs and cheap good quality implementations of open hardware are always available then we have succeeded.
So long as there is a community of people using and building our designs the designs will be available and constantly improving. If some people don't share the results of their efforts with us, then that's a shame, but we don't need them anyway. If they're that inclined to be selfish then their grudging cooperation probably wouldn't be that useful anyway.
I think we should go with the GPL for the moment. We can re-license later if we need to.
Tomorrow, I'll contact Eric Raymond, the OSI, and the FSF for advice.