One truism that we see shouted over and over again is "you can't make money in open-source". Of course if you are reading this then it is likely you don't accept that premise. But it is true to say that for many corporations it is not possible to make money out of open-source. In my $$job we do it all the time, we use Linux, we use open-source VHDL and Verilog all the time. It reduces our development cycles, gives us a broad base of support and reduces our costs, at least up front. But as our final products required significant compliance testing and such we could never produce open-hardware the software is an integral part of the operation.
However, take things like Gumstix, Arduino, Wiring, LEDuino, DAISY MP3 player and others proves that the concept of providing a hardware design that can be duplicated is growing. No one individual or even small group might be capable of exploiting a design, but maybe others can. The Economist recently did a piece on open-source which was quite positive. One of my personal motivations for designing open-source hardware is that I am not really a programmer! I can give you really good hardware, and maybe some basic code to run on it, but be darned if I want to spend my life maintaining and enhancing it!
If course I can make money selling hardware, even when I do provide all that is needed to replicate it. How many people really have the skill and resources to produce sophisticated electronics hardware? A bare PCB sold for $20 is cheaper than the time and effort to produce something yourself. So I can make my designs, make some pocket money out of them and you can have the fun of doing things on the boards, sound like fun?