It's not perfect, but this is cool: podscope - podcast search engine.
It seems to do a better-than-nothing speech-to-text index of podcasts, and provides a search interface on top of that.
For instance, here's a search on 'learning' - not perfect, but it provides a good starting point.
I love the functionality - takes you right to the appropriate portion of the audio file when you press the "play" widget.
I searched for some of the stuff I talked about in my last podcast, and it wasn't found - an indication that it's been so long since the podcast that it's trickled off my feed. Need to find some time to do some more...