This bug eluded me during several days because there was another bug that already caused VS 2010 Beta 2 to hang, but finally a couple of days ago I realized that was seeing a different issue (no commandbarbutton involved in this new bug) and I was able to isolate and report it today:

VS 2010 Beta 2 Bug: adding CommandBarPopup to CommandbarPopup causes VS to hang if CommandBarPopup was added to Toolbar before

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=507650

As I expressed some days ago, my feeling with VS 2010 so far (after four builds) is that once you patch your add-in to (somehow) bypass a bug, the add-in finds a new one.

The (VB.NET) code to reproduce it is:

Imports System
Imports Microsoft.VisualStudio.CommandBars
Imports Extensibility
Imports EnvDTE
Imports EnvDTE80

Public Class Connect
   Implements IDTExtensibility2

   Private _applicationObject As DTE2
   Private _addInInstance As AddIn
   Private _commandBarToolbar As CommandBar
   Private _commandBarPopupOnToolbar As CommandBarPopup
   Private _commandBarPopupMenu As CommandBarPopup
   Private _commandBarPopupOnMenu As CommandBarPopup

   Public Sub OnConnection(ByVal application As Object, ByVal connectMode As ext_ConnectMode, ByVal addInInst As Object, ByRef custom As Array) Implements IDTExtensibility2.OnConnection

      _applicationObject = CType(application, DTE2)
      _addInInstance = CType(addInInst, AddIn)

      Select Case connectMode

         Case ext_ConnectMode.ext_cm_AfterStartup
            InitializeAddIn()

         Case ext_ConnectMode.ext_cm_Startup
            ' OnStartupComplete will be called

      End Select

   End Sub

   Public Sub OnStartupComplete(ByRef custom As Array) Implements IDTExtensibility2.OnStartupComplete
      InitializeAddIn()
   End Sub

   Private Sub InitializeAddIn()

      Dim commandBars As CommandBars

      Try

         commandBars = CType(_applicationObject.CommandBars(), CommandBars)

         If MessageBox.Show("Create toolbar before menu?", "", MessageBoxButtons.YesNo) = DialogResult.Yes Then

            ' This hangs VS 2010 Beta 2
            CreateToolbar(commandBars)
            CreateMenu(commandBars)

         Else

            ' This causes nested menus but doesn’t hang VS 2010 Beta 2
            CreateMenu(commandBars)
            CreateToolbar(commandBars)

         End If

      Catch ex As Exception
         System.Windows.Forms.MessageBox.Show(ex.ToString)
      End Try

   End Sub

   Private Sub CreateToolbar(ByVal commandBars As CommandBars)

      _commandBarToolbar = commandBars.Add(Name:="My toolbar", Position:=MsoBarPosition.msoBarTop, Temporary:=True)
      _commandBarPopupOnToolbar = AddCommandBarPopupToCommandBar(_commandBarToolbar, "MyPopupOnToolbar", "My Popup On Toolbar")
      _commandBarToolbar.Visible = True

   End Sub

   Private Sub CreateMenu(ByVal commandBars As CommandBars)

      Dim menuCommandBar As CommandBar

      menuCommandBar = commandBars.Item("MenuBar")

      _commandBarPopupMenu = AddCommandBarPopupToCommandBar(menuCommandBar, "MyPopupMenu", "My Menu")
      _commandBarPopupOnMenu = AddCommandBarPopupToCommandBar(_commandBarPopupMenu.CommandBar, "MyPopupOnMenu", "My Popup On Menu")
      _commandBarPopupMenu.Visible = True

   End Sub

   Public Sub OnDisconnection(ByVal disconnectMode As ext_DisconnectMode, ByRef custom As Array) Implements IDTExtensibility2.OnDisconnection

      Try

         If Not _commandBarPopupOnMenu Is Nothing Then
            _commandBarPopupOnMenu.Delete()
         End If

         If Not _commandBarPopupOnToolbar Is Nothing Then
            _commandBarPopupOnToolbar.Delete()
         End If

         If Not _commandBarPopupMenu Is Nothing Then
            _commandBarPopupMenu.Delete()
         End If

         If Not _commandBarToolbar Is Nothing Then
            _commandBarToolbar.Delete()
         End If

      Catch ex As Exception
         System.Windows.Forms.MessageBox.Show(ex.ToString)
      End Try

   End Sub

   Public Sub OnAddInsUpdate(ByRef custom As Array) Implements IDTExtensibility2.OnAddInsUpdate
   End Sub

   Public Sub OnBeginShutdown(ByRef custom As Array) Implements IDTExtensibility2.OnBeginShutdown
   End Sub

   Friend Shared Function AddCommandBarPopupToCommandBar(ByVal objParentCommandBar As CommandBar, _
   ByVal sCommandBarName As String, ByVal sCaption As String) As CommandBarPopup

      Dim objCommandBarPopup As CommandBarPopup = Nothing
      Dim objCommandBar As CommandBar
      Dim objCommandBarControl As CommandBarControl
      Dim iPosition As Integer

      If Not (objParentCommandBar Is Nothing) Then

         iPosition = objParentCommandBar.Controls.Count + 1

         objCommandBarControl = objParentCommandBar.Controls.Add(Type:=MsoControlType.msoControlPopup, Before:=iPosition)

         objCommandBarPopup = CType(objCommandBarControl, CommandBarPopup)

         objCommandBar = objCommandBarPopup.CommandBar

         If Not (objCommandBar Is Nothing) Then

            objCommandBar.Name = sCommandBarName

            objCommandBarPopup.Caption = sCaption

         End If

      End If

      Return objCommandBarPopup

   End Function

End Class