Pheedo has launched an RSS / ATOM ad service. It has been tested over the past few months, and now launches with some high-profile publishers (including Lockergnome) and a variety of features:

• Sophisticated targeting and optimization capabilities. Unlike sending an email newsletter, consumers and aggregators are in control of requesting your feed. From every 15 minutes to once a day, consumers and aggregators pull your content feeds differently. We constantly monitor the feed to measure the noise-to-value ratio on the fly and insert ads when appropriate so as not to over-burden the consumer.
• Ad serving intelligence specifically built for RSS/Atom feeds. The system understands how each aggregator thinks and serves the appropriate ad based on the unique characteristics of the news reader.