This particular suicide bombing in Iraq merits a bit of analysis -
Suicide bomber kills 10 at Sufi Muslim gathering, Reuters. Does it matter whom the Islamists blow up?
Many people get that there're two large sects in Islam, the Sunnis and the Shi'i, and that the Shi'i are the majority in Iraq (and, post-war, in power) and in Iran.
There are other sects and one of the largest outside the main two is
Sufism, the mystical variant of Islam which emphasizes the spiritual connection with God and has adherents from both Shi'a and Sunni.
The Wahhabi Islamist movement that's been sparked by al Qaeda believes that all non-Wahhabi Sunni sects of Islam are blaspehmers and must be hunted down and destroyed. In Iraq, their current political motive is to rile up an internal Islamic religious civil war, pitting the strict Sunni interpretation of Islam versus all others and in particular, the Shi'i, who're in power in the new Iraqi government. In Pakistan, the sectarian strife that the Islamists are causing is a direct reflection of the country's political situation, where Musharraf has the unpleasant job of trying to balance appearing strong and kowtowing to the overwhelming pro-Islamist / retroactive Sunni majority.
This Iraqi suicide bombing and the suicide bombing of a Sufi gathering in Pakistan
last Friday could mean a few things: the Islamists are losing ground, having to strike out at a lesser publicly (Western) known sect of Islam or, alternatively, the Islamists are showing their supporters that they're committed to their version of Islam by rooting out apostasy in their midst. I'd argue that it's the latter, Islamists distinguishing themselves by striking out at any "unbelievers" is a reaffirmation of their ultimate goals for structure: a new Caliphate under their version of Shairi'a. Further, adding Sufis to their hit list (since they're still killing Shi'a ulema when they can get their hands on them, in Iraq) is a way to get the governments in question to protect this group and therefor implicate themselves in heresy, bringing more cause to overthrowing those in power.
They're not slapping randomly, they're building a case by attempting to back established governments into a corner where they can further denounce their legitimacy and the legitimacy of those that support the governments.