"Assuming the above is true, then why do we not see any sign of these probes?" Because for example these probes are as tiny as a nano-meter? Hey it is the year 2012 here on earth. In the last decades we have already shrunk computers from the size of a entire room to fit in our pockets and soon our blood. And we ourselves can already build nano-technology, despite not being able to travel around our galaxy. What tells us that intelligent life has to build "probes" so big that we are able to detect them. Maybe these probes are just as large a nanometer and are already flying towards us or were near us or are already on Pluto. Or maybe other life forms shrink themselves to fit into a trillionth of a trillionth of the diameter large spaceship to be able to travel around space faster than light. All of the theories are just expanding from current knowledge as of today, not factoring in any stunning surprises of how the universe might really work :)
This seems like a good point on the surface, but since it takes only one outlier to dominate the universe (by Bostrom's train of thought), then it would have happened anyway. It's the law of smart for one, dumb for all. Assuming most life forms would decide to live minified, any life form not deciding so would have immense advantage to consume all untapped resources. Following this logic, and since any sufficiently advanced replicators would all be competing for the same space, the most overwhelming kind would take over all others.
