Rafat Ali on the challenge that Podcasting presents for advertisers:

Podcasting Presents Ad Challenge: The same portability that makes podcasting appealing makes it difficult to track with meaningful metrics beyond the number of downloads. That’s one of the obstacles podcasters will have to overcome to take podcasting to a level of advertising beyond experimentation. Pheedo CMO Bill Flitter told ClickZ some advertisers are trying custom 800 numbers or offer codes to get a handle on the response rate from podcasts. Pheedo has a 30-podcast ad network that has drawn six advertisers so far. FeedBurner is trying the same technology it uses to track RSS feed readership.

Still, what’s most likely to push podcasting as an ad medium are the moves into the space by Apple and other major companies.