...
BEGIN_MSG_MAP_EX(CNewDialog)
...
NOTIFY_HANDLER_EX(IDC_NAMEEDIT, EN_CHANGE, OnNameChange)
...
END_MSG_MAP()
...

LRESULT OnNameChange(LPNMHDR lpNMHDR)
{
  CEdit wnd = GetDlgItem(IDC_NAMEEDIT);
  int iStrLen = wnd.GetWindowTextLength();
  ...
   UIEnable(IDOK, iStrLen != 0, true);

  return 0;
}

E_>Привожу кусок карты сообщений диалога, в котором реализована сия идея. Не работает . Я потому и спрашивать стал.

E_>...
E_>BEGIN_MSG_MAP_EX(CNewDialog)
E_>...
E_>NOTIFY_HANDLER_EX(IDC_NAMEEDIT, EN_CHANGE, OnNameChange)
E_>...
E_>END_MSG_MAP()
E_>...

E_>LRESULT OnNameChange(LPNMHDR lpNMHDR)
E_>{
E_>  CEdit wnd = GetDlgItem(IDC_NAMEEDIT);
E_>  int iStrLen = wnd.GetWindowTextLength();
E_>  ...
E_>   UIEnable(IDOK, iStrLen != 0, true);

E_>  return 0;
E_>}

E_>

    BEGIN_MSG_MAP(CMainDlg)
      COMMAND_HANDLER(IDC_NAMEEDIT, EN_CHANGE, OnEnChangeNameedit)
   END_MSG_MAP()

   LRESULT OnEnChangeNameedit(WORD /*wNotifyCode*/, WORD /*wID*/, HWND /*hWndCtl*/, BOOL& /*bHandled*/);

LRESULT CMainDlg::OnEnChangeNameedit(WORD /*wNotifyCode*/, WORD /*wID*/, HWND /*hWndCtl*/, BOOL& /*bHandled*/)
{
   // TODO:  If this is a RICHEDIT control, the control will not
   // send this notification unless you override the CAxDialogImpl<CMainDlg>::OnInitDialog()
   // function and call CRichEditCtrl().SetEventMask()
   // with the ENM_CHANGE flag ORed into the mask.

   // TODO:  Add your control notification handler code here
   ATLTRACE("OnEnChangeNameedit()\n");

   return 0;
}