eMarketer cites andquestions a new report, "Podcasting Catches On," by Pew, estimating that more than six million American adults - 29 percent of the 22 million U.S. adults who own MP3 players - have listened to a podcast.

Pew admits the numbers are probably overblown, due to a "broad" survey question. But the potential audience is upscale, which should help attract advertisers.

Some 18 percent of households with annual income of more than $75,000 have MP3 or iPod devices.