This is a remarkably persistent complaint. The source of the leak was ToolStrip installing an event handler for the SystemEvents.UserPreferenceChanged event. So that it can respond to the user changing the theme or color scheme and redraw itself. This is a static event, forgetting to unregister the event handler will permanently leak the ToolStrip instance.
The bug has definitely been fixed in .NET 3.5 SP1. The ToolStrip.Dispose() method unregisters the event handler. If that's the version you are running, make sure that the Dispose() method indeed runs. A common mistake is to use Controls.Remove() to remove a control from a form but then forgetting to call Dispose() on the removed control.