Well, after lots of digging and running into dead ends I went back and reinstalled the WP-iPodCatter plugin for WordPress. I had done that yesterday and it didn’t seem to be working until I added “enclosure” to the Custom Fields area of the WP write panel. Once I did that, I could easily add the URL of the m4a file and it gets dropped into the feed. I can then use the itpc:// protocol to let users auto-subscribe to my podcast feed via iTunes.
I have it working here at Learning and Innovation as well as my class blog. It seems like a step backwards from the way WP 1.5 just added the file, but I could be wrong about all this. At any rate using the plugin, adding the custom field, and dropping in the full URL of the file made it all work.