IANAL either, but I think the prohibition is on items that threatan life and saftely rather than trespassers property.
From Wikipedia:
> Mantraps that use deadly force are illegal in the United States, and there have been notable tort law cases where the trespasser has successfully sued the property owner for damages caused by the mantrap. There is also the possibility that such traps could endanger emergency service personnel such as firefighters who must forcefully enter such buildings during emergencies. As noted in the important US court case of Katko v. Briney, "the law has always placed a higher value upon human safety than upon mere rights of property."
Thinking in those terms, a few spike strips on your lawn should be fine.
Unless someone walks across the lawn, steps on a spike, and develops tetanus. The point is it might be legal but you run the risk that it could blow back pretty badly.
From Wikipedia:
> Mantraps that use deadly force are illegal in the United States, and there have been notable tort law cases where the trespasser has successfully sued the property owner for damages caused by the mantrap. There is also the possibility that such traps could endanger emergency service personnel such as firefighters who must forcefully enter such buildings during emergencies. As noted in the important US court case of Katko v. Briney, "the law has always placed a higher value upon human safety than upon mere rights of property."
Thinking in those terms, a few spike strips on your lawn should be fine.