[CraigBuchek]

Another possible explanation is that interstellar travel is impossible. Perhaps there's too much debris between star systems, and it's not possible to create materials strong enough to make a spaceship that can withstand a collision with the debris at the speeds required.

This doesn't quite explain the lack of communication signals. But if nobody can colonize the galaxy, then there would be less sources of signals.

BTW, the Wikipedia article on the Fermi Paradox (http://en.wikipedia.org/wiki/Fermi_paradox) does a very good job of detailing all the possible explanations.

[akkartik]

In a world where interstellar travel is impossible, civilizations can't strike out too far for resources; they must make do with what is nearby. So as they get more and more efficient with their energy usage they'll emit less and less. Compare our solar system with a dyson sphere the size of the solar system, for example.

[kordless]

I'm rather fond of a variant of the "we aren't listening properly" argument. If there exists a medium of which we are unaware, and which can be used to communicate effectively at large distances, perhaps interstellar travel is simply unnecessary, or overly dangerous.

[stcredzero]

Just one percent of C is awfully fast, but a crawl on interstellar scales. Still, it would be possible to colonize other stars even at such a pace. We can certainly envision accelerating a mass up to that velocity along with a spacecraft in its wake.