Stratfor makes a couple of interesting observations about the Bin Laden media blitz following the death of Al-Zarkawi. First, the fact that Bin Laden and Mullah Omar used very different formats and presentation styles suggests that the two were not in the same place at the time these messages were made. Second, the text of Bin Laden's message essentially boils down to a message to the Sunni in Iraq to continue the violence against the Shi'a. Bin Laden doesn't usually comment on failures (he has ignored the deaths of several senior associates in Saudi Arabia), but in this case he seems to be afraid that the Iraqi's are going to rally around the government and find a political solution to their differences.
The fact that the Iraqi cabinet was formalized on the same day as Al-Zarkawi's death may be one more reason for Bin Laden to be nervous. Perhaps Al-Zarkawi's head was part of a quid pro quo connected to the distribution of powers?