[jhonny]

Sacados de Trucomania:

Para ocultarla:

ShowWindow(Application.Handle, SW_HIDE);

Para mostrarla:

ShowWindow(Application.Handle, SW_SHOW);

Para Windows 2000

procedure TfrmMain.BtnHideFromTaskBarClick(Sender: TObject);
begin 
  ShowWindow( Application.Handle, SW_HIDE );
  SetWindowLong( Application.Handle, GWL_EXSTYLE,
                 GetWindowLong(Application.Handle, GWL_EXSTYLE) or 
                 WS_EX_TOOLWINDOW and not WS_EX_APPWINDOW);
  ShowWindow( Application.Handle, SW_SHOW );
end;

Espero te sirvan.