This is a guest post by Aseem Kishore, a technology enthusiast and lead blogger for Online-Tech-Tips.

The US podcasting market has steadily been increasing in size over the last couple of years and is expected to double within the next two years to almost 38 million people, according to eMarketer. With these kinds of numbers, podcasters and advertisers are starting to look into ways to generate more revenue through video ads.

It is estimated that the total US podcast audience for 2007 is 18.5 million with an active podcast audience of 6.5 million. The 18.5 million number refers to individuals who have ever downloaded a podcast from anywhere. The "active" user base refers to individuals who have downloaded an average of one or more podcasts per week.

The active podcast audience is expected to grow to an impressive 25 million listeners by 2012 and advertisers are expected to spend close to $450 million by 2012.

There are many reasons why podcasting is becoming more popular in the US at such a fast rate. Firstly, podcast content is easy to digest. Most podcasts that are downloaded are usually short and focus on specific topics or genres, making it easy for people to listen to exactly what they are interested in and skip the fluff. With professional high-quality content and no commercials, it's easy to see why people may prefer to view a podcast over a TV show.

Also, there is a much greater awareness and much more promotion for podcasting via large online news and media networks such as CNET, New York Times, and well-respected blogs [Ed: see our own ReadWriteTalk for example]. Podcasts have also become part of many people's daily lives thanks to Apple's iPod and iTunes. With iTunes and the vast numbers of iPods, users are able to instantly and easily download podcasts onto their PC's or iPods, making watching a podcast almost as normal as listening to an MP3.

Following the success of Apple's iTunes, other major players have released portable media players that can play podcasts - such as Microsoft's Zune player and the Zune Marketplace - thereby increasing the podcast audience.

On top of that, there has a been a prediction that within four years 40% of all video viewed will be outside of the television set. That's coming from a poll carried out by Jack Meyers and tracked by the firm Teletrax of 300 media executives.

The key questions will be whether the growth of the podcast audience will continue to increase at the current rate and will it be enough to attract large amounts of ad spending. Most podcasters make money through advertising on the pages/feeds that distribute the podcast, as well as embedding ads into the podcast itself. Sponsorships and subscription services are not as popular, but will grow as the podcasting market increases.