Preinstalled applications are in /system/app folder. User installed applications are in /data/app. I guess you can't access unless you have a rooted phone.
I don't have a rooted phone here but try this code out:
public class Testing extends Activity {
private static final String TAG = "TEST";
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
File appsDir = new File("/data/app");
String[] files = appsDir.list();
for (int i = 0 ; i < files.length ; i++ ) {
Log.d(TAG, "File: "+files[i]);
}
}
It does lists the apks in my rooted htc magic and in the emu.