There are two things going on here, that are very different:
Apple cannot own the word "pod." Sorry guys, you built your brand on a regular 'ol common word. Apple derived a benefit from using an existing word as part of their product name and they can't just turn that around and take the whole damn word for themselves. They could have called it the "iKerbock" you know, and they'd now own "kerbock." But then, they wouldn't have sold as many iKerbocks as iPods, I'd bet.
However, iPod is Apple's word and they should clearly be able to stop people from using it, especially in an mp3 playing context.
podcast - public domain word
iPodcast - infriges on Apple's trademark.
To put it another way: If Apple can decide to own the word pod, what other words can be owned?