Pretty good discussion here:
http://www.freerepublic.com/focus/news/769517/posts
This is a post civil war law (not a constitutional issue) that has a lot of loopholes, if someone needed to exploit it. For example, as I understand it, the Navy doesn't apply, so they could send in the Marines.
It could be repealed, as well, but Tom Daschle would want UN approval first.