for anyone else looking for that, its:
vb Code:
Imports Extensibility
Imports EnvDTE
Imports EnvDTE80
Private Sub ClearImmediateWindow()
Dim dteObject As EnvDTE.DTE = System.Runtime.InteropServices.Marshal.GetActiveObject("VisualStudio.DTE.9.0") 'vb2008
Dim currentActiveWindow As Window = dteObject.ActiveWindow
dteObject.Windows.Item("{ECB7191A-597B-41F5-9843-03A4CF275DDE}").Activate() 'Immediate Window
dteObject.ExecuteCommand("Edit.SelectAll")
dteObject.ExecuteCommand("Edit.ClearAll")
currentActiveWindow.Activate()
End Sub