Is there a third-party ad placement plugin or utility for WordPress that works with Yahoo APT, or do I have to roll my own? It looks like there are a lot of options for Adsense, but I can't seem to find anything for APT.
Use AdRotate. This is not specific for AdSense, but let you define blocks that you can insert in your template.
In this blocks, you can show the adsense/yahoo/png/flash advertising that you want. It manage the rotation automatically. If there are mora than an ads in a block, it display at random!
Stats included