.....the Non-linear battlefield may have front lines, but they will be circular, and will be around pockets of strong enemy influence, as has already been detailed. The real difference is that while there are front lines, there no safe rear areas, since lines of communication and supply are often the guerilla's preferred targets. Therefore one of the primary objectives of the counter-guerilla forces is to prevent the guerilla harassing the lines of supply and communication.
The vast majority of US military personnel are traveling around Iraq in versions of the HMMWV light truck or HMMWV Armament carrier. Insurgents know that HMMWVs usually contain Americans and therefore they are priority targets for any aggression. I don't have any specific figures, but browsing the news reports suggest that a very significant proportion of our losses have been from personnel traveling in HMMWVs.
The M113 (unofficially known as the Gavin) is the preferred platform since its large roof hatch provides good situational awareness and allows an armed Soldier to survey each quadrant. The weight of the M113 is about the same as that of a civilian truck so the vehicle will encounter few problems with weak bridges. I believe there are about a thousand M113s being used currently in Iraq and they are proving to be very successful.
In urban situations the M113's better armour and situational awareness make it harder to attack. It has the capability to climb across improvised barricades such as parked cars. The M113 is similar in length and width to a HMMWV but can turn on the spot, so is less likely to get trapped in narrow streets.