I've refused permission for some stories -- that is, I've both stated publicly and replied when asked that I prefer that certain stories not be podficced. As with most things fannish, the effectiveness of the author's preferences is moot.

I think it's polite to ask for permission, or at least, to let the author know a recording is being made. I think that a podfic version of a story is a transformation of the original work into a different medium and not the equivalent of text-to-speech.