Sistani's timing is probably not an accident
I suspect Allawi and the USMC commanders want Sistani or one of Sistani's trusted people to come up to the gates of the Imam Ali mosque and tell Sadr's mob to leave. That leaves Sadr with no viable options. Sadr can't dynamite the mosque at that point. The ultimate objective is to erase Sadr's threat as a demagogue. Getting Sistani into the mix is the best solution to a thorny problem.
Gary