Found this:
Every time GarageBand exports a song, it first creates an uncompressed mixdown file, called Output.aif. It then compresses this file into the file that you actually see. If you ever want to access the raw, uncompressed file, you can do this:
1. Initiate the export. GarageBand says it's creating the mixdown.
2. Open up Activity Monitor (Applications » Utilities » Activity Monitor) and select the GarageBand process.
3. Press the Inspect button at the top.
4. Select the Open Files and Ports tab. Copy the contents of it into a text editor.
5. Search the text for Output; it's usually on the last line.
6. Copy the path to the clipboard -- it will be something long and ugly like /private/var/ folders/8c/8c6KjuqYGKCLxYlDtiJFJk+++TI/ -Tmp-/GarageBand/Export/Output.aif (robg says: Note that spaces were added to the path shown here to allow line breaks; the real path won't have spaces, of course.).
7. Open up Terminal (Applications » Utilities » Terminal). Enter mv, press the Space Bar, then press Command-V to paste the copied path, then press Space Bar one more time. Do not press Return yet.
8. Drag the folder in which you wish to save the raw mixdown file into the Terminal window, then press Return.
9. Wait for GarageBand to finish creating the mixdown.
That's it! As you moved the Output.aif file, GarageBand doesn't know where it is, so when it tries to compress the file, it'll fail. Therefore, the compressed M4A file won't be created.
I'm going to try this when I get home tonight.