I've been a longtime supporter of Ogg Vorbis, and I'm also a Mac user. While I haven't figured out how to get my iPod to play Ogg just yet, I have worked on getting iTunes to play it. I periodically do searches to see if things have improved, but they seem to return mostly old data.
So I just wanted to get it out that the good people at Xiph have started maintaining the Ogg Vorbis plugin. It is available here.
I don't seem to be able to find the page again, but I thought I read there were some small problems with the last release. They have development snapshots here. At least so far, I haven't run into any problems with it. And overall it seems to consume fewer CPU resources than the older releases.