This conversion can be performed in both
Amadeus Lite ($24.99 from the
Apple App Store only - requires OSX 10.5.5) and
Amadeus Pro which is $40 and will work on OSX 10.4 or higher, including Snow Leopard, and has more facilities. The latter is available in a trial version, the former isn't.
To convert, just drag the podcast episode out to the Desktop and then drag it onto Amadeus's icon. It will open it as an audio file (only) and it can then be saved as an MP3.
(And before anyone mentions the free program Audacity, it won't do this.)