Of course - for unconstitutional laws are not laws at all!
This applies to enforcing laws as well. The soldiers who were "just following orders"when they illegaly seized the weapons owned by katrina survivors and left them at the mercy of looters are morally responsible for their actions.
From: http://en.wikipedia.org/wiki/Constitutionality
"The legal encyclopedia
American Jurisprudence says the following in regard to constitutionality:
The general rule is that an unconstitutional statute, though having
the form and the name of law, is in reality no law, but is wholly void
and ineffective for any purpose since unconstitutionality dates from
the time of its enactment and not merely from the date of the decision
so branding it; an unconstitutional law, in legal contemplation, is as
inoperative as if it had never been passed ... An unconstitutional law
is void. (16 Am. Jur. 2d, Sec. 178)"
-Robur