A basic nuclear rocket—the kind we could build today—can reach 10 kilometers per second. That's gets a robotic factory across the galaxy in 1.5 billion years. The oldest habitable stars are considerably older than that.

Top quality propulsion can shrink the time to 50 million years, which means somebody can colonize the galaxy faster than dinosaurs can evolve into talking monkeys.

So there has been plenty of time for robotic probes to colonize the galaxy. So where are they?

The time between dinosaurs and talking monkeys is basically irrelevant. The important figure is the time between the formation of the sun and talking monkeys - 4.5 billion years. You're also making some assumptions about how easy it is to build a ship capable of crossing interstellar distances and also reproducing itself.

In fact, using current technology, we actually could build a "self-replicating probe" loaded with algae and other photosynthetic single-celled organisms. So we really can't rule out ourselves as the evidence we're looking for. We might even be evidence that such probes usually malfunction.

We are an interstellar malfunction.

I like that.

Minor nit: is there evidence that dinosaurs evolved into monkeys? didn't they all get wiped out by an asteroid?

AFAIK, mammals existed during the dinosaur era. But they occupied a tiny ecological niche and were physically tiny. They survived the asteroid hit because they were tiny.

The absence of dinos enabled them to eventually occupy niches occupied by dinos.

Dinosaurs are birds.

"So there has been plenty of time for robotic probes to colonize the galaxy. So where are they?"

They're bacteria and they already colonized Earth.

A word for the idea that simple life may be everywhere, and likely initially reached Earth from elsewhere, is 'Panspermia':

http://en.wikipedia.org/wiki/Panspermia

My hunch is that it's true: that when we get better at detecting it, we'll discover bacteria/viruses/spores of various kinds all over the place (including Mars and other solar system locales).

And perhaps less seriously speaking, I always thought this type of virus looks like a robot: http://www.cosmosmagazine.com/files/imagecache/feature/files...