Here's a GREAT article on "nullification." My favorite quote from the article: "Federal actions taken outside the scope of federal power are not laws, they are acts of usurpation. In other words, an unconstitutional federal "law" is no law at all, and the states and their people are not bound to obey them." [emphasis mine]

http://www.tenthamendmentcenter.com/2010/11/13/unconstitutional-law-is-an-oxymoron/

Yes, and when the general government and its agents act or attempt to act under those usurpations, they are not acting lawfully but merely under the color of law.  States have an obligation and the authority to interpose themselves between these usurpations and the people of the respective state.  One instrument of interposition is the instrument of nullification.