Exporting as .aiff

When you share the song as an audio file to iTunes you can save it uncompressed as 16bit AIFF file at 44.1 kHz.

See this Help Text: http://help.apple.com/garageband/ipad/2.0/index.html#chs39284d66

Send a GarageBand song to iTunes

• Tap My Songs, then tap Select.
• Tap the song you want to send, then tap the Action button .
• Tap Share Song via iTunes.
• Do one of the following:
  • To send the song as a GarageBand song, tap GarageBand.
  • To send the song as an audio file, tap iTunes, then tap Share.
  After you tap the iTunes icon, you can choose the quality of the file GarageBand sends to iTunes. You can also add artist, composer, and title information that appears in iTunes.
• Sync your iPad with your computer.In iTunes, the exported song appears in the Documents list in the File Sharing area when GarageBand is selected.
• Do one of the following:
  • Drag the song from the GarageBand Documents list to the Finder.
  • Click Save As, navigate to the location where you want to save the song, then click Save.

You can open the GarageBand song in GarageBand on your Mac, or play the audio file in iTunes (or any compatible audio application) on your computer. GarageBand songs cannot be opened on a Windows computer.