Although unlikely, I came up with a possible variation on the berserker probe scenario to help explain the Fermi Paradox. A berserker probe is like a Von Neumann probe, except that it destroys life instead of spawning it. It's conceivable that, if a civilization can create a life-spawning probe, an advanced civilization could -- for whatever reason -- also create its antithesis: an exponentially duplicating fleet of malevolent probes designed to wipe out all intelligent life in the galaxy. Of course, we haven't seen any signs of berserker probes, so we are left wondering why our planet hasn't been wiped out by them. There are a number of plausible answers to this problem, including the popular Rare Earth hypothesis.
However, a possible explanation for why Earth still has life on it in the presence of berserker probes is that berserker probes, in order to remain energy efficient, remain dormant in space awaiting activation by a sign of intelligent life. One can assume that berserker probes are designed to eliminate advanced intelligences only. What sign of intelligence could they be waiting for? How about radio signals. Yikes.
This is very unlikely, however, for a number of reasons. Primarily, it would seem that we are about a half-century away from Drexlerian nanotechnology, artificial superintelligence, and an existential phase shift (i.e. technological singularity). By cosmological standards, if a berserker probe is going to wipe us out, it better get here already. And assuming that it was lying dormant in our solar system, it has had ample time to destroy us. Moreover, it would have been simpler to design a fleet of berserker probes that permanently sterilize all planets in the galaxy so that life can never arise.
At the very least, this might make an interesting premise for a science fiction story.