This is really interesting research: "Stealthy Dopant-Level Hardware Trojans." Basically, you can tamper with a logic gate to be either stuck-on or stuck-off by changing the doping of one transistor. This sort of sabotage is undetectable by functional testing or optical inspection. And it can be done at mask generation - very late in the design process - since it does not require adding circuits, changing the circuit layout, or anything else. All this makes it really hard to detect.
The paper talks about several uses for this type of sabotage, but the most interesting - and devastating - is to modify a chip's random number generator. This technique could, for example, reduce the amount of entropy in Intel's hardware random number generator from 128 bits to 32 bits. This could be done without triggering any of the built-in self-tests, without disabling any of the built-in self-tests, and without failing any randomness tests.
via Schneier on Security: Surreptitiously Tampering with Computer Chips.