Try this just to see if it works.  Use a timer to re-execute the activation of the form.  The problem could be that all of this is trying to execute on the same thread and so the re-activation never takes place.  Also, this is not an issue with SF, it has 100% to do with the way WinForms works.

InfoBox()
'-- You can reduce this to a smaller interval, but starting with 1 second will give you a good visual when  it executes.
MyTimer.Interval = 1000
MyTimer.Enabled = True

Timer Code
MyTimer.Enabled = False

'-- You might try just forcing the focus back to the control itself first.  You could then try
'   All of the other options previously mentioned in this thread if it doesn't work.
MyControlToFocus.Focus()

Thanks!!! It worked ok you got the solution, I not suppose that a timer can change the things, but you clarify me it.

Dim InfoboxTimer As New Timer()
InfoboxTimer.Interval = 50 'the min value that work ok for me...
AddHandler InfoboxTimer.Tick, AddressOf InfoboxTimer_Tick
InfoboxTimer.Enabled = True
MicroFour.StrataFrame.Messaging.InfoBox.DefaultTimeOut = 1000
MicroFour.StrataFrame.Messaging.InfoBox.NotifyBox("Title","description")

'Timer_Tick  handler
Private Sub InfoboxTimer_Tick(ByVal sender As Object, ByVal e As EventArgs)
      Dim Tmr As Timer = CType(sender, Timer)
      Tmr.Enabled = False
      Me.ParentForm.Activate() 'I coding on a user control... activate his form
      'Me.txtCodigo.Focus()
End Sub