In such a hot political environment it's bound to happen. Some news say militants have denied involvement, but some reports say intercepted Al Qaeda calls indicate their involvement.
However, I find it strange that Al-Qaeda doesn't take credit for it but they'll take credit for attacks, like the recent bombing in Algeria.
In Addition: Jan. 4, 2008. Al-Qaeda claims responsibility for the shooting of a U.S. diplomat and his driver in Sudan.
Update: According to an Italian newspaper, an Al-Qaeda spokesman, Mustafa Abu al-Yezid, claimed the group's responsibility for the attack on Benazir Bhutto.
Baitullah Mehsud, a Taliban commander in South Waziristan, denied involvement after the Pakistani government supplied an taped conversation with an unknown man on the day of Bhutto's death. The allegedly incriminating tape does not mention Benazir Bhutto however.