[delfib]

Yo lo hago por medio de un TAction que al agregarlo en el ActionList le pongo el mismo nombre de la forma con prefijo act por ejemplo actMiForma, además en la forma al final del código le agrego

Código Delphi [-]
initialization
  RegisterClass(TMiForma)
finalization
  UnregisterClass(TMiForma)

Aunque esto ya lo automatice por medio de un wizard, si te interesa puedes bajar mi demás código de http://sourceforge.net/projects/delfib

Código Delphi [-]
unit uaxActionMenu;

interface

uses
  Forms, SysUtils, Classes, Controls, ActnList;

type
  TaxActionMenu = class(TCustomAction)
    procedure actAbrirVentanaExecute(Sender: TObject);
  private
    FModal: Boolean;
    FOnmrOK: TNotifyEvent;
    function CrearVentana(sForma: string): TForm;
    procedure AbrirVentana(sForma: string);
    procedure SetModal(const Value: Boolean);
    procedure domrOK;
    procedure SetOnmrOK(const Value: TNotifyEvent);
  public
    constructor Create(AOwner: TComponent); override;
    function Execute: Boolean; override;
  published
    property Caption;
    property Name;
    property ShortCut;
    property Modal: Boolean read FModal write SetModal default False;
    property OnmrOK: TNotifyEvent read FOnmrOK write SetOnmrOK;
  end;

implementation

{ TaxActionMenu }

function TaxActionMenu.CrearVentana(sForma: string): TForm;
var
  FormClass: TFormClass;
begin
  sForma:= StringReplace(sForma, 'act', 'frm',[]);
  Result:= TForm(Application.FindComponent(sForma));
  if not Assigned(Result) then
  begin
    FormClass:= TFormClass(GetClass('T' + sForma));
    if Assigned(FormClass) then
       Result:= FormClass.Create(Application);
  end;
end;

procedure TaxActionMenu.AbrirVentana(sForma: string);
begin
  if FModal then
  begin
    if CrearVentana(sForma).ShowModal = mrOK then
      domrOK
  end
  else
    CrearVentana(sForma).Show
end;

procedure TaxActionMenu.actAbrirVentanaExecute(Sender: TObject);
begin
  inherited;
  AbrirVentana(TAction(Sender).Name)
end;

constructor TaxActionMenu.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FModal:= False;
  OnExecute:= actAbrirVentanaExecute
end;

function TaxActionMenu.Execute: Boolean;
begin
  Result:= inherited Execute
end;

procedure TaxActionMenu.domrOK;
begin
  if Assigned(OnmrOK) then
    OnmrOK(Self)
end;

procedure TaxActionMenu.SetModal(const Value: Boolean);
begin
  FModal := Value;
end;

procedure TaxActionMenu.SetOnmrOK(const Value: TNotifyEvent);
begin
  FOnmrOK := Value;
end;

end.

Código Delphi [-]
unit uaxActionMenuReg;

interface

uses
  SysUtils, Classes, ActnList, uaxActionMenu;

procedure Register;

implementation

procedure Register;
begin
  RegisterActions('Menu', [TaxActionMenu], nil);
end;

end.