The U.S. Army currently plans to keep about 100,000 troops in Iraq through 2010. It also wants to send troops to "overseas hardship posts" (like Iraq, Korea or Afghanistan) only once every three years. In theory, that should be possible. There are some 150,000 troops in those hardship (where you can't take your family, and are likely to be shot at) assignments, and 500,000 active duty troops (plus over half a million reservists who can be called up for a year every few years)..
There's a problem with that, because some 42 percent of the 500,000 active duty troops have never been to Iraq, while 20 percent have been there more than once. In some job categories, like Special Forces, troops have spent more than half the last five years in places like Iraq and Afghanistan. While the army also has some 600,000 reservists, they also have legal restrictions on how much they can be activated. And for morale (and recruiting) purposes, you don't want to send them into harm's (or discomfort's) way too often.
Actually, the math isn't too bad. About twenty percent of the active duty army, for various reason (health, pregnancy, or a rare job skill), cannot be sent to a hardship post. That means the actual shortfall is manageable. In theory. The army plan is to have more civilians take over jobs done stateside by troops, and retrain more troops for combat zone jobs. That, and more cash bonuses for those taking on an exceptional number of overseas deployments, are expected to solve the problem, or at least reduce the harmful effects.