I was just messing around with some of the various options for recording a podcast, and think this solution will work quite nicely: Snapz Pro X 2.0. It is usually intended to be used as a screen recorder, but also does an excellent job of recording both system sound and microphone input. I set it to record a 1x1 screen capture movie, and discard the video track when saving. Then, the .mov file is brought into iTunes, where I convert it to MP3. (set the encoding options properly, and the Advanced menu contains a "Convert selection to MP3" option.
It looks like Snapz Pro will let me record the incoming audio from iChat (or Skype, or whatever), and mix it with the microphone input, and any other system sounds (like iTunes, or any other source...)
Anyway, if anyone is looking for an easy podcast recording system, this looks pretty good. I've got no idea how it works in the field yet, but it worked great in the tests that I ran (just remember to use headphones, so the mic doesn't pick up the system audio as well)
UPDATE: I just did some testing, and it looks like it's possible to use freeware for the whole recording/publishing system. I just used Rogue Amoeba's LineIn program - it is an app that lets you pass the input from the microphone through the speakers, and Ambrosia Software's WireTap 1.0 which lets you record all audio to a file. The combo works pretty well, and it's free. WireTap 2.0 was wrapped into Ambrosia's Snapz Pro 2.0 product, which explains why it does such a nice job of recording all system audio. If the free stuff turns out to not work so well, I'll go with Snapz Pro 2.0
In my little experiments, I've found it really hard not to sound like some brain-dead radio DJ, especially with my voice echoing in my headphones (trick there - get real headphones, or turn down the volume so you can hardly hear it... oh, and remember to talk normally... ;-) )