[cpollock]

I just did a web search and found a lot of info on this. The ceramic cap displays some microphonics, and is allegedly more nonlinear than the plastic film, so it is not typically used for audio coupling. The plastic caps do not couple mechanical energy into a voltage, and have less nonlinearity, so they are preferred for audio coupling applications.
I had no idea. I always though ceramic would be the most stable, but clearly I was wrong. Anyway, it sounds like the best coupling caps are film caps, and for power line filtering it probably doesn't matter either way because any microphonics that might appear will be completely shorted by the low impedance of the power line.
Sorry to add this noise to the forum. Other than my input I'm really enjoying reading the stories posted here.