New Al Qaeda video

Al Jazeera has aired a video with Al-Zawahri, Al Qaeda's second in command. He states that Blair had brought the London bombings upon the country.

If Al Zawahri and Bin Laden are hiding out together it is a very interesting question why it is Zawahri is on the video, not bin Laden. Is bin Laden sick? Is he perfecting his image - preserving the shock effect of seeing videos with him?