OnItemClickListener не работает?

Я думаю, что вы не использовали метод viewholder(), он отлично работает, когда я его проверил. См. Полный код:

public class FirstScreen extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        ListView lv=(ListView) findViewById(R.id.ListView01);


        MyAdapter adp=new MyAdapter(getApplicationContext());

        for (int i = 0; i < 50; i++) {
            adp.addItem("item " + i);
        }
        lv.setAdapter(adp);
       // lv.setItemsCanFocus(true);

       lv.setOnItemClickListener(new OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,
                    long arg3) {
                // TODO Auto-generated method stub
                showToast(arg2);
            }
        });
        //registerForContextMenu(lv);

    }

    public void showToast(int arg)
    {
        Toast.makeText(this, "Position"+arg, Toast.LENGTH_SHORT).show();
    }

     class MyAdapter extends BaseAdapter {

        private Context mcontext;
        private ArrayList<String> aList=new ArrayList<String>();    

         public MyAdapter(Context context) { 
             mcontext = context;          
        } 

        @Override
        public int getCount() {
            // TODO Auto-generated method stub
            return aList.size();

        }

        @Override
        public Object getItem(int position) {
            // TODO Auto-generated method stub
            return aList.get(position);
        }

        @Override
        public long getItemId(int position) {
            // TODO Auto-generated method stub
            return position;
        }
         public void addItem(final String item) {
             aList.add(item);
             //notifyDataSetChanged();
         }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {

             // TODO Auto-generated method stub
             LayoutInflater inflater = (LayoutInflater)mcontext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

             convertView=inflater.inflate(R.layout.list_row,parent,false);


             ((TextView)convertView.findViewById(R.id.txt1)).setText(getItem(position).toString());
             ((ImageView)convertView.findViewById(R.id.img1)).setImageResource(R.drawable.icon);

             return convertView;
        }

        }

     static class ViewHolder {
         TextView   tab1Text;
         ImageView  backgrouundView;
    }

}

main.xml и list_row здесь не включены.