The Yahoo Publisher Network (YPN) now supports having their ads placed in feeds (RSS, ATOM). It looks like only Movable Type and WordPress are currently supported. Instructions for adding ads to your feeds are provided, but the images they used are barely readable. The WordPress screen shots are bordering on useless they are so difficult to read. Somebody should have caught that before these instructions were posted.
I’ve added the YPN ad feed code to the feeds for this blog to try this out. If these ads work in a reasonable way then Yahoo may have taken a big step forward over Google AdSense by supporting WordPress in addition to Movable Type.