I think the presense of armed federal troops and/or the U.S. military in non-federal areas establishes martial law, regardless of whether or not it has been 'officially' declared. Could be wrong.
You are, but that's okay. It isn't the presence that causes the legal condition of "martial law" to exist. It is the extent to which civil libery has been suspended that indicates "martial law," and even then, there are degrees of suspension.
Therefore, the mandatory evacuation declared by Nagin estabishes a sort of martial law - people are ordered out, and not permitted in - that is, the liberty to come and go unimpeded is removed. State imposed condition, and although in LA the term is "State of Emergency," it amounts to martial law.