The idea of imperial overstretch is that the demands placed on a "hegemon" (a dominant power) can be so great that they can endanger the very existence of the hegemon. I do not believe that the US is in any imminent danger of having this happen.
American spending on the military accounts for about 20% of the federal budget. This is, of course, a very large percent, but surely not enough to bring down the country. That amount will also (hopefully) decline to some extent as we pull out of Iraq and, perhaps, Afghanistan.
If it were not for the fact that we find ourselves in this economic crisis (which had nothing to do with our status as a hegemon), we would not be worrying about the amount of money we are spending on our military.