I just found this at Informed Comment:

The USG Open Source Center sent out the following note, which points out that in scouring jihadi web sites, the analysts have found none where al-Qaeda has taken credit for assassinating Benazir Bhutto. You think of the announcement by al-Qaeda posted the morning of 7/7/05 when the London Underground was bombed. Yet they are not claiming this one. Curioser and curioser:

"FYI - - No Al-Qa'ida Claims for Bhutto Assassination Found on Jihadist Websites
Jihadist Websites -- OSC Summary
Monday, December 31,

FYI: No Al-Qa'ida Claims for Bhutto Assassination Found on Jihadist Websites

As of 1830 GMT on 31 December, OSC has not observed any official statements from Al-Qa'ida claiming responsibility for the assassination of Benazir Bhutto on any of the websites that normally disseminate authenticated statements from the group. OSC will continue to monitor these websites for any such claims."
http://www.juancole.com/2008/01/pakistan-elections-postponed-ppp.html

An al-Qaeda member known as Mustafa Abu al-Yazid is said to have claimed the assassination for al-Qaeda shortly after it was carried out. This claim is usually sourced to an Italian news site. However, the Italian website appears to contain no original reporting of this claim, although it does carry a report of a phone call made by al-Yazid to the Asia Times:
http://www.adnkronos.com/IGN/Esteri/?id=1.0.1712687460

You can find the Asia Times article this appears to be based on here:
http://www.atimes.com/atimes/South_Asia/IL29Df01.html

There appears to be a conflict between what the man claiming to be al-Yazid told the AT and al-Qaeda's web-silence on the issue. I am motivated to ask: how did the Asia Times know it was really al-Yazid? Was it not unwise for al-Yazid to call the Asia Times, given the NSA can monitor and trace calls? He's in Afghanistan and must have used a satellite phone and the NSA must be all over satellite phones in Afghanistan. There's also the question of how and why al-Yazid had her shot and well as bombed and then induced the Pakistani military to lean on doctors not to give the real cause of death - bullet wound not concussion.

Al-Yazid was also reported to be under house arrest in Iran a year or two after 9/11.
http://www.msnbc.msn.com/id/8330976/
I wonder about the circumstances of his escape.

I note that the unconfirmed claim from al-Yazid was enough for lots of media organisations to report that al-Qaeda did it. Just check this:
http://www.cnn.com/2007/WORLD/meast/12/27/bhutto.dhs.alqaeda/index.html
It's a report by CNN about the claim, although the body of the report casts doubt of al-Yazid's claim, it's entitled " Officials: Al Qaeda claims responsibility for Bhutto killing." It is not hard to see who would benefit from this type of claim.

btw, for afficianadoes of Saeed Sheikh here is an article of former Alec Station chief Michael Scheuer about al-Yazid. Can you spot the howler?
http://www.jamestown.org/terrorism/news/article.php?articleid=2373518