The major problem with IEDs is not the cleverness of their builders and techniques for avoiding them. American troops quickly developed techniques to find IEDs and destroy them. No, the big problem is that it takes a lot of troops and equipment (especially helicopters and UAVs) to keep roads "clean" of IEDs. The most heavily used routes, like the MSR (Main Supply Route from Kuwaiti ports into Iraq) is kept clean, as are many other heavily used roads. But patrols have to cover thousands of kilometers of roads and there are not enough resources to keep all roads "clean." It's the small patrols or reaction forces that rush down a little used road that are at most risk. The IED users are known to have set up, and taken down, hundreds of IEDs because no coalition troops came by. But when a hummer or truck does roll by, without a prior reconnaissance by engineers or MPs, an IED is successful. Many of these secondary routes are narrow roads twisting through residential areas. This means the vehicles have to slow down and makes them more vulnerable to IEDs. The IED users are not afraid of hurting nearby civilians if the locals are not firmly in the anti-Saddam camp.
This game of can and mouse will continue until the new Iraqi government establishes sufficient police control over the anti-government (mainly Sunni Arab) areas to make it too difficult to plant IEDs (without a high risk of getting caught.) Each IED that explodes near coalition vehicles causes, on average, 2-3 casualties.
The most effective weapons the anti-government forces in Iraq have used are roadside bombs (called IEDs, or improvised explosive devices.) Nearly 3,000 IEDs are known to have been used since last May. The IEDs are basically booby traps using remote detonation devices (garage door openers, wireless doorbells, cell phones, walkie talkies, Etc.) or wired detonation. Explosives range from military plastic explosives (C4), to commercial explosives (dynamite) and explosives taken from bombs and artillery shells. These are basically the "booby traps" that were widely used during World War II. After that war, most armies equipped their troops with detonators and mechanical devices (for trip wires) so that they could more easily make booby traps themselves. Troops were also trained to deal with enemy booby traps. Roadside bombs are the alternative to improvised bombs under the road, and are used in Iraq because most of the roads used by coalition troops are paved (which makes it pretty obvious where a bomb is buried.) Sometimes the bombs are placed in pipes or culverts that go under the road, and operate like the old "command mine" (a mine that is set off by an operator, not a pressure device on the mine.) But most IEDs are on the side of the road.