How can I change the default location for bluetooth file transfers? I have an Android 2.3 device (Samsung Galaxy Y Duos, S6102) and want all files that I receive via bluetooth to save to my SD card.

How can I do that?
The default location is already on your external SD card in the /bluetooth folder.

It's not possible to configure it up to Android 4.1 Jelly Bean:
With the included stock bluetooth receiver you cannot change the location as of now, because it's hard coded. You need a 3rd party app like Bluetooth file transfer which might be configurable here.

In ./packages/apps/Bluetooth/src/com/android/bluetooth/opp/ you can see it.

Constants.java defines this:  

public static final String DEFAULT_STORE_SUBDIR = "/bluetooth";


BluetoothOppReceiveFileInfo.java has no switch for the location:

File receiveFile = new File(uniqueFileName);
if (sDesiredStoragePath == null) {
    sDesiredStoragePath = Environment.getExternalStorageDirectory().getPath() +
        Constants.DEFAULT_STORE_SUBDIR;
}