We've improved our Transistor's Dynamic Ad Insertion system works:
You can now run multiple campaigns simultaneously
You can choose to enable/disable a campaign on specific episodes
Various user interface improvements
Also, if you don't want a particular episode to get assigned to a campaign, even the default, you can set the campaign in your episode settings to "None."
On Transistor, podcasters are using Dynamic Ads in all sorts of creative ways (here's an example).