From Juan Cole:

Al-Sharq al-Awsat reports that [Arabic URL] its contacts in the radical fundamentalist community of British Muslims maintain that the July 7 attacks on London were undertaken by one of several al-Qaeda sleeper cells in Europe, which had been planted there by Ayman al-Zawahiri and his lieutenants in preparation for a decades-long war with the West.

If this is true, then responsibility for the bombings can ultimately be traced back to the guys who brought us 9/11 -- not merely to affiliates using the al-Qaeda name. Note that that's Zawahiri, bin Laden's #2, not Zarqawi, who's causing trouble in Iraq.