Open-source designs (for software or hardware) aren't a complete solution. You need to regularly audit the end-result, lest the compiler/fab add unwanted nasties to your design.
OR we might want to go back to shopping anonymously "offline", hoping that the NSA will not bother backdooring every device on the market.
There's some research [2] into using authenticated, encrypted bitstreams, but even if the implementation matches the theory (and after all, it's crypto, we know how that goes...) this only reaches the same level of security as a fixed-configuration ASIC, since FPGAs are vulnerable to the same nefarious fab attacks as ASICs.
1: http://www.cmpe.boun.edu.tr/caslab/publications/selfreconf_f...
Also, just transitioning to a more open process with an open codebase has the effect of "keeping them honest". Once the code is out there, they have no way of knowing who might be scrutinizing it and have no chance to retroactively cover something up.
This is why it's such a big deal when companies make a commitment to openness (the real kind -- not the buzzword kind). It's a statement of being willing to forego most of the dirty tactics that being closed source allows and to playing a fair(er) game.