Today, I downloaded Cast Away, which is an applescript that allows you to specify settings for each podcast in iTunes. For example, you can specify how many episodes to keep for podcast.
It looks like it can do this cleanup on a daily basis. When you set this up, all that it does is an a entry in your user's crontab.
Unfortunately, I wasn't able to get this to work. I was getting some AppleScript error. Until I get this to work, I have reverted to manually deleting podcast episodes that I don't have time to listen to.