While you can't export directly to Audacity, you can get pretty close using a workaround. The secret is to freeze all of your tracks using Garageband's temporary-rendering feature. Internally, this means that all of your tracks are being rendered, individually, as AIFF files.
You can get to these files by ctrl+clicking the .band file and choosing "View Package Contents." Somewhere in the folder hierarchy is a folder called Freeze Files, which contains the AIFF recordings of your last freeze. Copy these to another folder, and import them into Audacity as separate tracks.
You can also import them into any other audio software that allows AIFF import, which is a pretty common feature.