This practice of moving troops and equipment as separate entities is a Cold War innovation. To speed the movement of reinforcements from the United States to Europe, in the event that the Soviet Union invaded, two divisions had one set of equipment in Europe, and another set back in the United States, where they were based.. Actually, the plan was also developed as a politically acceptable way to withdraw two divisions from Europe. This was done in 1968, but the equipment stayed behind, and was stored and maintained by contractors (German civilians). Starting in 1969, some of the troops would fly to Europe, fire up the gear, and go out on field exercises. The troops would then return the gear to the storage areas and fly home. These annual exercises lasted until 1988. The experience gained in all those REFORGER exercises made the army, and marines, confident that they could apply the concept of pre-positioned equipment elsewhere. This also led to the idea, as applied in Iraq, of having the first units to be there, to leave their gear behind (if they were being replaced by the same type units.) This saved a lot of money in shipping costs, not to mention the additional work the troops had to do preparing everything for sea movement.
As units pull out of Iraq, and are not replaced, they will win the lottery, so to speak, and take the equipment with them (with the exception of some gear that will probably be given to the new Iraqi armed forces.) Since units in Iraq have first dibs in new gear, as well as spare parts, the equipment in Iraq is often the latest, and well maintained. Yes, it is used a lot, and theres a lot of wear and tear. But the stuff the last unit takes home will be battle tested and up to date.
Depending on what sort of treaties are negotiated with the new Iraqi government, some pre-positioned equipment for American combat units may stay in Iraq. Iran is still seen as a threat to Iraq, and just the presence of pre-positioned American equipment is often enough to give frisky neighbors pause. Kuwait has been the host to several combat brigades worth of American equipment for over a decade. Again, that provides more practical experience about maintaining the stuff in a desert environment. There are also lots of Kuwaitis who have experience maintaining the gear.
Another enormous troop movement is just about finished in Iraq. The second annual troop rotation to Iraq is almost complete. Some 15,000 troops were kept in Iraq to provide additional security for the elections last month. This turned out to be a prudent move, as the security was good enough to keep the vast majority of voting places safe. But now the 18th Airborne Corps headquarters, the 42nd Infantry Division (a National Guard outfit), the 3rd Infantry Division and the 2nd Marine MEU (Division) have replaced the 3rd Corps headquarters, the 1st Infantry Division, the 1st Cavalry Division and the 1st Marine MEU (Division). American troops strength will go down to 138,000 by March. The increased traffic from over 250,000 troops moving in and out over the last few months has caused an increase in traffic accidents, although combat injuries have been declining since the battle of Fallujah last November. Most of the heavy equipment belonging to units remains in Iraq, to ease up the logistics burden. The troops either return to the equipment they left behind when they came over, or are issued new equipment and weapons, to the replace those being left behind, when they get home.