The two sources of armed resistance, the Sunni Arab nationalists and Baath Party Saddam supporters, and the Shia Islamic radicals, are both having details of their plans, and support, revealed by captured documents and personnel. The Fallujah resistance is largely former Saddam security functionaries fearful of their fate once the American troops leave. But there are also lots of Arab nationalists (who are still looking for medieval crusaders to fight) that are being allowed to move through Syria to get into Iraq. Syria is the only other country in the world run by the Baath Party (an Arab nationalist-socialist movement). For decades, the parties in Syria and Iraq were bitter enemies, but now the Syrian Baathists are fearful of ending up like their fellow Baathists in Iraq.
Shia firebrand Moqtada al Sadr also has foreign connections, but from the other direction, in Iran. Sadr's charitable operations are funded and "supervised" by Iranian police and security organizations. This is the "America is the Great Satan and Must Be Destroyed" crowd that held the American embassy staff captive in Iran for over a year in 1979. Most Iranians have moved beyond this sort of thing, but a hard core, with veto power over the Iranian government, have not. The Iranian Islamic radicals are Sadrs bankers and advisors. This was another reason why most other Iraqi Shia clerics kept their distance from him. Iraqi Shia do not want to be dominated by Iran (which is run by Indo-European "Persians" who are considered, by Arabs, as bad as the Indo-Europeans from the west.) Iran denies everything, but the evidence is piling up that proves otherwise.
The Sadr group denies having anything to do with the kidnapping of aid workers. It appears that this is being done by the Baathists, who are offering bounties for criminal gangs (lots of those around) to make the snatch and turn them over to the former secret policemen for further processing. The kidnapping has had the desired effect of slowing down many relief and reconstruction projects, as foreign relief workers stay off the streets until the current uprising is put down.
About a thousand American troops ran Sadrs militiamen out of Kut, two days after the Sadr gunmen had chased Ukrainian troops out. The Ukrainian troops are low quality, largely conscripts and sent to Iraqi by the corrupt Ukrainian government to earn money and favor from the United States. The other European contingents in the south were composed of well trained and led soldiers who have been able to deal with the Sadr gunmen.
Neither the Sunni Arab or Sadr gunmen have proved very capable against American troops. Superior tactics, training and equipment, have enabled U.S. troops to systematically destroy the armed groups they encounter in the cities. While 46 American troops have died in the past week, that's less than a tenth of the number of Iraqi gunmen who have died. Moreover, U.S. troops are using minimal firepower to limit civilian casualties. The Iraqi gunmen try to take advantage of this, by moving and firing from behind human shields (women and children are preferred). This doesn't work, and just gets women and children killed. It's tough and tedious work for the U.S. troops, but it's ultimately fatal for the Iraqi opposition.