The highest levels are in units serving in Iraq. From Stragegypage:
In the last six months, the U.S. Army is seeing 15 percent more soldiers re-enlist than expected. This continues a trend that began in 2001. Every year since then, the rate at which existing soldiers have re-enlisted has increased. This despite the fact that 69 percent of the troops killed in Iraq have been from the army. New recruits continue to exceed join up at higher rates as well.
All this is extremely important, especially when there is a war going on. Experience saves lives in combat, and more of the most experienced troops are staying in. This means that, a decade from now, the army will have a large and experienced corps of senior NCOs. That, in turn, means the younger troops are likely to well trained and led.
The army makes a big thing, internally, about the number of troops re-enlisting, especially within combat units that are in Iraq or Afghanistan. Pictures of mass re-enlistments are published in military media, but the civilian media has generally ignored this phenomena. Also ignored, except by some local media interviewing locals who are in the army, is the positive attitude of the troops, especially those in combat units. The large number of re-enlistments occur because the troops believe they are making a difference, and winning. This is especially true for soldiers who have come back to Iraq on a second tour, and noted the improvements since the first tour.
Go read the whole thing.