[1] http://www.actel.com/products/pa3/ [2] http://www.actel.com/documents/pa3_faq.html
(I guess there is no real advantage in keeping this obscured)
Who's to say that manufacturing in China means the backdoor was injected by China? I would have thought the US is just as likely a source, given that the design came from there. Surely the US government would love having access to FPGAs in foreign systems?
The cynic in me says that Cambridge needs to keep poking, as they might find two backdoors: one inserted by the US, the other by China.
Power glitch detection, mechanisms to detect decapping/stripping, wire mesh shielding, protection against ultra-violet laser stimulation of transistors, ... are all important.
For those interested in further reading, Security Engineering[1] by Ross Anderson contains a section on chip security. Another paper[2] by Ross Anderson and Markus Kuhn (1996) provides additional background.
I can envision a scenario where this "backdoor" is actually part of the designed-in security features of the chip designed to prevent an unauthorized party from reading out the FPGA "programming" as it were. As such, it's conceivable that there might be multiple keys or even a series of "transport" or "default" keys that are similar to those found on ISO smartcards. What we might be looking at is a "feature" as opposed to a "backdoor."
In any case, this sort of thing only becomes a critical security breach if the application you're using the chip in depends on periodic (or boot-time) reprogramming of the FPGA. In either case, either the physical security or the trust chain of your firmware loads is broken. As we all know, key management and side channel attacks are the hardest part of implementing a secure crypto system, so is this really news?