Garage Band 3 from the iLife 06' suite is a fabulous App. I use it daily, to create this podcast. It provides Jingles, Sound efx, voice tracks, and all kinds of features to really make podcasting easy. However, one thing it doesn't do so well... allow flexibility with exporting. By default, if you choose 'Create a new Podcast Episode', it adds several tracks, including a 'Podcast' track. When this track is present, it will export the song to iTunes in AAC format, instead of the preferred uncompressed AIFF file. If you export it as AIFF, then you can convert it to whatever format you want, which will help keep additional fidelity.
Here is how to do it:

Choose 'New Podcast Episode'

To be able to export as 'AIFF', you need to delete this track.

Go to 'Track -> Delete Track' and then it will remove the podcast track. Now, when you go to 'Share -> send song to iTunes', it will now export it in 'AIFF' format, instead of the compressed AAC. Once it is in iTunes, then you can convert it to your preferred format for distribution.