Predicts it will reach $150 million by 2008.

Podcasting has a long ways to go before it becomes a mainstream medium, but is poised to grow exponentially through the end of the current decade and will increasingly become an attractive outlet for advertisers, says a new report from eMarketer.

By 2008, podcast advertising should reach $150 million, approaching $300 million by 2010, according to the report.

Currently, despite all the hype surrounding the nascent medium, eMarketer estimates that just 3 million Americans are active podcast listeners, and a total of 10 million people have ever downloaded a podcast to date.

And advertising on podcasts, "is in its infancy to put it mildly," says Mike Chapman, eMarketer editorial director, who authored the report. "Podcasting is not set to become a new mass-market venue, at least for the next half decade. By way of comparison, U.S. broadcast radio still reaches close to 200 million Americans," he said.

However, the number of podcast listeners is expected to skyrocket over the next four years, reaching 50 million total listeners by 2010, with 15 million or so listeners being described as active - i.e. downloading podcasts on a weekly basis.

Part of the rationale behind these growth estimates is rapid adoption seen for the medium just in the past couple of years - as total listening levels rise by 400 percent since 2004.

"The total audience for podcasts has shown meteoric growth, particularly in the US," said Chapman. "The active listening audience is much smaller, but it is still set to grow rapidly."

That volatile growth has made predicting just where podcasting will end up difficult, acknowledges eMarketer's report, and some estimates forecast that the podcast listening audience could be as large as 80 million by decade's end.

Of course, just a few years ago, few people had actually heard of podcasts, which are audio (and sometimes video) file which owners of MP3 players like Apple's iPod download to consume whenever and whenever they'd like. The podcasting landscape still remains mostly populated by small-audience amateur radio shows, but traditional media companies ranging from VH1 to NPR to CBS have added podcasting content to their media output over the past year.

Part of the reason that the podcasting has yet to go mainstream is due to the still small number of MP3 players in the market--with some reports estimating that 11 percent of Americans own such devices. Also, awareness among the general population of just exactly what a podcast is is relatively low--though its much larger amongst young adults and early adopters, says eMarketer.

It's those audiences that have made podcasting attractive to several big name advertisers, despite its small usage numbers. Already, auto brands like Volvo, Toyota and Honda are sponsoring podcasts, and the report predicts that since "podcasting is positioned to deliver highly specialized content to niche groups," that all sorts of advertisers are likely to tap into the medium.

At this point, there are no publically available estimates on the podcasting ad market, according to the report, but several new companies have popped up recently, including Audible and Podtrac, which promise to improve ad spending metrics for the medium.