I'd be interested in knowing that as well.
Another poster also mentioned the posse commitatus as being something that would prevent the deployment of troops on the borders. Well, that the law has essentially been gutted by the "war on drugs" types over the past couple of decades. Also, if protecting our borders isn't a legitimate role of the military, then I don't know what is.
Just a couple weeks ago there were American troops on the New Mexico/Mexico border in an observation/enforcement support role. That has gone on sporadically for a long time. They rotate in for occassional training missions. I'm sure there were threads on Free Republic about it and I know it was discussed on Bill O'Reilly and Hannity and Colmes and elsewhere in the media. That argument is largely a canard. A fundamental mission of our military is to protect our borders.