That's a silly observation to make. If some foreign country invaded my neighborhood today, I don't even own an "identifiable uniform" to wear while I sniped at them from my bedroom window. Would the Geneva Convention find it more acceptable for me to wear my hockey team's colors than my pajamas?
I'd also point out that under these standards it would also be a "war crime" for the U.S. to have private security contractors operating in a place like Iraq. I'm sure those guys aren't dressed like the typical security guard at an office building in New York or Chicago.
The appropriate portion is here (emphasis mine):
Article 4
A. Prisoners of war, in the sense of the present Convention, are persons belonging to one of the following categories, who have fallen into the power of the enemy:
1. Members of the armed forces of a Party to the conflict as well as members of militias or volunteer corps forming part of such armed forces.
2. Members of other militias and members of other volunteer corps, including those of organized resistance movements, belonging to a Party to the conflict and operating in or outside their own territory, even if this territory is occupied, provided that such militias or volunteer corps, including such organized resistance movements, fulfil the following conditions:
(a) That of being commanded by a person responsible for his subordinates;
(b) That of having a fixed distinctive sign recognizable at a distance; (c) That of carrying arms openly;
(d) That of conducting their operations in accordance with the laws and customs of war.
3. Members of regular armed forces who profess allegiance to a government or an authority not recognized by the Detaining Power.
4. Persons who accompany the armed forces without actually being members thereof, such as civilian members of military aircraft crews, war correspondents, supply contractors, members of labour units or of services responsible for the welfare of the armed forces, provided that they have received authorization from the armed forces which they accompany, who shall provide them for that purpose with an identity card similar to the annexed model.
5. Members of crews, including masters, pilots and apprentices, of the merchant marine and the crews of civil aircraft of the Parties to the conflict, who do not benefit by more favourable treatment under any other provisions of international law.
6. Inhabitants of a non-occupied territory, who on the approach of the enemy spontaneously take up arms to resist the invading forces, without having had time to form themselves into regular armed units, provided they carry arms openly and respect the laws and customs of war.
He left out the carrying arms openly part.
"Insurgents" as in Iraq tend to hide their weapons behind clothing or other objects so as to appear as unarmed civilians until such a time as they spring their traps; they deliberately select firing positions from within crowds of unarmed people of all sexes and ages so as to prevent themselves from standing out in a crowd. Or they simply hide their bombs in objects designed to look civilian. They don't put their insignia or other identifying marks on car bombs.
US civilian contractors carry their arms openly, do not select firing positions inside crowds of civilians for their ambushes, and do not use civilians to protect themselves by taking advantage of the enemy's reluctance to kill civilians.
The last one is a no-brainer since none of our enemies have any reluctance to kill civilians.
Citizen militias are expected to carry arms openly and not fire from beneath the skirts of women or from inside schools and churches and mosques simultaneously occupied by noncombatants. Even if uniforms are unavailable they are legitmate combatants so long as weapons are carried openly.