Except that with the proliferation of laws in recent decades, "Ignorance of the law is no excuse" has become a sick joke. It is literally impossible for someone to know all the laws they could be breaking. See for example Harvey Silverglate's recent book, Three Felonies a Day.
I personally think this statement is pretty misunderstood. I don't think they mean "ignorance of the law is not a valid excuse" (i.e. "you should have known") but rather "if you don't know the law we're still going to punish you" (i.e. "we don't care if you could have known or not").
Also, apparently ignorance of the law is perfectly acceptable if you are a cop or DA, http://reason.com/archives/2010/08/02/ignorance-of-the-law-i...