Atrios:

Suspected Sunni Arab gunmen killed 23 policemen Sunday, including 17 in one attack in the predominantly Shiite southern city of Basra, signaling the possible start of an intensified insurgent campaign against Iraq’s predominantly Shiite Muslim security forces.

Political tension deepened in Baghdad when Vice President Tariq al-Hashemi, the country’s highest-ranking Sunni politician, threatened to resign if Prime Minister Nouri al-Maliki did not act quickly to eradicate two feared Shiite militias.

I'm sure it's just pre-election violence.