The format's not the problem. When your prefs say to burn mp3 CDs and you burn a playlist with non-mp3 tracks, iTunes gives you an option to burn without 'em or cancel. Can you burn regular audio CDs? Can you burn an mp3 CD from a playlist with no tracks in common?
Of course, if the podcast is a non-mp3 file, you should burn it as audio anyway, so Jacke's point is still useful