The BBC and Al Jazeera are reporting that fighting has broken out between the Iraqi army and Moqtada al-Sadr's Mahdi Army, as well as other Shi'a militias.
It's looking like a real flashpoint:

Al-Maliki is said to be personally overseeing the operation in Basra against the Mahdi Army, a British army spokesman said.
Major Tom Holloway said: "The prime minister came down to Basra from Baghdad yesterday along with a delegation. He is overseeing the operation. He is at an Iraqi military base."
James Bays, Al Jazeera's correspondent in Iraq, said: "[The British forces] are sort of on standby if they are really needed … they could help out this operation in a number of ways, with air support for this operation, either with bombing or surveillance useful to Iraqi forces on the ground."
"There are groups of competing militia on the ground. The main ones are al-Sadr's fighters … but there are lots of other splinter groups. On top of that there are criminal gangs and smugglers in Basra."
Al-Sadr has instructed the Mahdi Army to maintain its ceasefire across Iraq in recent weeks.
However, the US military and Iraqi security forces have detained al-Sadr loyalists they accuse of belonging to breakaway armed factions.
Al-Sadr said that his group could lead a nationwide campaign of civil disobedience in response to the Iraqi security forces' operation in Basra.
He also said he would launch a nationwide strike unless attacks against members of his movement cease.

If you feel, as many do, that Sadr's cease-fire has played a large role in reducing violence in Iraq during the surge period, then the battle in Basra has far-reaching implications for the entire country. A recent missile attack in the Green Zone originated in Sadr City, according to the Christian Science Monitor, showing further signs of tension. Plus:

Over the past two weeks fighting mainly between Iraqi forces and elements of the Shiite militia has flared in several parts of Baghdad and areas to the south of the capital.
US military spokesman Maj. Mark Cheadle says that Mr. Sadr's freeze on attacks continues and labeled those conducting acts of violence as "rebels," suggesting that they were rogue elements not obeying Sadr's orders. "Logic dictates that these groups cannot be part of his organization," says Major Cheadle.

I'm not sure that "logic" should be our only evidence here, though. "Logic" might dictate that the Mahdi Army is testing the waters with low-level attacks, or that pressures which begin at the bottom could eventually sway al-Sadr's thinking at the top.
As I've said before, many eyes are on Basra, attempting to gauge whether the withdrawal of British forces there is working. Today's events make for pessimism. But do they mean that America should stay in Iraq for another 100 years? In part, the recent violence may indicate that the US can only delay the inevitable struggle for power, not prevent it. Without diplomatic initiatives and a real change in mentality, all we're doing at best is plugging holes in a collapsing dam. No military power can make fantasy into reality forever.
Moreover, when we act in bad faith we're only making the situation worse. Are we holding people in detention unfairly? Are we skimping on paying our allies? Maybe another factor that makes renewed violence unavoidable is our own behavior, and the behavior of the corrupt puppet government in Iraq. The Mahdi Army and the Awakenings militia cannot be expected to continue supporting us if we don't hold up our end of various bargains.
As fighting continues, and the Iraqi army seals off Basra, we may witness the resumption of full-scale civil war in the country, as well as some grim choices about what we're doing there.
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