I'm not so sure. Much of the ISAF force in Afghanistan is made up of non-US troops - including Canadians. The American role there is mostly in 'mopping up' and chasing after the stragglers of al-Queda and the Taliban.
Iraq is a different playing field, though. But, as was mentioned before, the US is withdrawing troops from Europe, as well as in East Asia (with reduced committments in Korea and Japan), and that frees up substancial forces for possible rotation through Iraq. A large number of Army and Marine reserve units have still not been activated.
Recruiting officers have said that they have noticed a drop in enlistment numbers than, say, the period immediately after 9/11, but they're still well within their quotas and necessary replacement numbers.
As I said before, the military itself doesn't see a need for a draft and actually opposes it, because it would lower the overall quality of their troops. I agree that the war in Iraq will be won with boots on the ground, but maintaining a high quality of soldier is necessary, especially in Iraq. And, the quantity of available US troops (Army, Marines and National Guard units) is far larger than what is currently deployed in either of the two countries right now...