It seems there are plenty of licenses already, GPL, GFDL, CC-BY-SA, attempts at hardware licenses such as TAPR Open Hardware License.
But hardware means physical goods, and physical goods have many properties that are very different from software. Many licenses come from a background of digital goods. The purpose of a copyleft hardware license should not be to tie people down, force them into certain behavior. It's more important to write down the guidelines and principles on which people who believe in copyleft hardware operate. Similar to Debian's Social Contract with the Free Software Community.
So from having met with and talked to many copyleft hardware pioneers, I will write down some principles and behavior that I have seen them valuing and following in their decision making. Over time maybe this can become a properly worded document, right now I am just jotting down my thoughts since Jon forced me to...
 Social Contract with the Free Software Community
- Software on our devices will remain 100% free
We promise that hardware we sell will only run free software as per the FSF guidelines (did they finally publish something?). We will support people who create or use both free and non-free works on our hardware. We will never make the system require the use of a non-free component.
- be friendly to people that want to use your equipment
- support alternative/recyling/non-commercial projects
- software is downloadable and can thus be multiplied at negligible cost. hardware cannot. but we can share hardware, a lot. Huge amounts of expensive test gear are locked away in labs and unused 90% of the time. Sharism.
- offer hardware with same terms for everybody, same pricing
- shipping, reach people in remote areas, fair shipping prices
- value the inventors and creators, patent system broken. How to promote the work of the copyleft hardware pioneers, how to credit them? advertisement clause?
- access to internal findings, empirical data, research results, test setups
- sharism for expensive tools, scopes, spectrum analyzers, protocol analyzers, laser cutters, test chambers, x-ray machines. copyleft hardware company should be like hacklab.
- access to EVBs, prototypes, unused parts. Loan stuff, ask it back, or after finish send to the next person. Be careful with stuff, keep everything in good shape.
- production should use free software as well
- we acknowledge the need to use proprietary software in some places, such as IC design, board-level design, mechanical design, test equipment. Will work on free replacements over time, but not jeopardize today's hardware production.