Charlie Stross presents an alternate explanation in the novel 'Accelerando'. Advanced civilizations may evolve at an accelerating pace, so anyone moving away from the central star (and source of energy) won't be able to catch up.

Interesting. I'll have to add that to my reading list.

There's also the possibility that signals just merge into the background after a few light years. So they are saying that our radio signals have already travelled a few decades, but they also diffuse under the 1/r^2 law. If there were an identical to Earth, how close would it have to be for SETI to pick up the radio chatter?

It's also possible that aliens use higher-bandwidth optical or above signals in order to communicate. Already as the Earth's networks swell with bandwidth, most of it is coming from optical installations rather than radio links. Even radio is relatively low-powered, so it's unlikely a lot of it is visibly leaking into space.

theory: background radiation is actually alien messages. The sky is completely filled with it.

The reason we can't read it as a signal is not because it has diffused, but because it's compressed so effectively that, for us, it is indistinguishable from noise.