At a BBQ this weekend I talked with Brian and Catherine, owners of Sounds Publishing NSFW, which publishes audio erotica. Brian told me that only a tiny percentage of podcasts are erotic ... but it's by far the most requested/searched for genre.
Violet Blue's Open Source Sex podcast is one of the most popular, and she also keeps a directory of other sexy podcasts.
Stay tuned to Sex Drive for an interview with Violet about her latest book, The Smart Girl's Guide to Porn.