Maybe, it will be one of the harder exploits to code, but very effective to disable certain apps or hide other backdoors. Reprogramming the microcode would be a virus-writer's dream come true, too. Knowing how to do it is probably very hard, as it is probably some RTL.

Intel started releasing reprogrammable microcode after the FDIV bug in the original Pentium made them replace a few hundred thousand CPUs. With programmable microcode, they could replace this operator.