I suspect that though in many ways Osama bin Laden (when alive) disdained Saddam Hussein, there has been significant cooperation between them (particularly with regard to human resources). Each is united to the other by a hatred for America; each knows that they might have to call on the other for support in the future. The 9/11 attacks could easily have brought forth an attack on Iraq. (It was considered by the president at the time - and it has indeed led to a likely confrontation with Iraq.) I imagine that bin Laden may have wished to inform Hussein beforehand, as a gesture of 'respect' (i.e., potential future need), so that Hussein would be ready for any immediate American response.