The Washington Independent (H/T Spencer Ackerman) has the rollup of current US Army force strength (including a really good chart). Before I begin, let me define some terminology. There's been a huge debate at Tom Ricks' "The Best Defense" blog regarding what "combat troops" are in this era of asymetric war and non-contiguous battlefields. Using the term "combat troops" and "support troops" is somewhat of a misnomer, as even cooks and lawyers have been shot at and returned fire during operations in Iraq and Afghanistan. For the purposes of this entry, however, we will use the term "combat troops" to refer to troops who serve in brigade combat teams, as opposed to aviation brigades or support brigades. (See also "combat brigades")
According to the Washington Independent, there will be some 50,000 troops in 14 active-duty
brigade combat teams available for reinforcements in Afghanistan, based on a 12-month "dwell time" at home for rest, refitting, and re-training. However, of those 50,000 active-duty troops, 19,000 are in "heavy" brigade combat teams. That means they fight in tanks and Bradley armored fighting vehicles, which are unsuited for Afghanistan. The 30,000 remaining troops are a mix of light brigades and "Stryker" brigades--using the Stryker wheeled armored vehicle for transportation.
Light infantry units can be deployed to either Iraq or Afghanistan, but heavy units have only been used in Iraq. While serving in Iraq, most brigades fight as "motorized infantry"--trading their usual vehicles for up-armored HMMWVs and MRAPs--so there really isn't much of a difference. Afghanistan is a different story--only light and Stryker brigades have participated, leaving about 1/3 of the force unsuited for combat operations.
Solution? Re-train tankers and mech guys as infantrymen. I'll admit that I'm recycling this story, but The Capatain's Journal had a great piece about physically training mechanized infantrymen--who normally ride in Bradleys--for operations in Afghanistan by conditioning them to 50-mile long excursions in the Appalachians.