The Under the Radar blog has two write ups on two different ad networks that have sprung up to place ads in podcasts.

Kiptronic:

Sponsorship network engine to help podcasters find sponsors and incorporate them into their podcasts. Kiptronic enables podcasters to take advantage of advertising opportunities without giving up control of content. The Kiptronic Podcast Marketplace features insertion technology that allows podcasters to use the ads they want without being too intrusive to the podcast. Podcasters don't have to manually insert ads into their content because Kiptronic automates the process. Advertisers can access targeted podcasts tailored for their specific message and in a geographically appropriate location.

Podbridge:

Podbridge says they can measure exact number of audience listens to a audio or video podcast, not just downloads. Also, audience profiles with demographics, listening habits and locations. Ads include spots from BBC and Clear Channel. Most ads are 15 seconds or less. Ads are sold throught Ronning Lipset Radio, an ad rep. "For podcasting to become a mainstream medium, it needs a business model to encourage content providers," Navar said to ClickZ. "We aim to help our content partners generate revenue by providing the insight and accountability needed to attract advertisers."