Programmatically go back to the previous fragment in the backstack

Look at the getFragmentManager().popBackStack() methods (there are several to choose from)

http://developer.android.com/reference/android/app/FragmentManager.html#popBackStack()

To elaborate on the other answers provided, this is my solution (placed in an Activity):

@Override
public void onBackPressed(){
    FragmentManager fm = getFragmentManager();
    if (fm.getBackStackEntryCount() > 0) {
        Log.i("MainActivity", "popping backstack");
        fm.popBackStack();
    } else {
        Log.i("MainActivity", "nothing on backstack, calling super");
        super.onBackPressed();  
    }
}