Can a Constitutional Amendment be Unconstitutional?

A lawsuit challenging the constitutionality of California's constitutional amendment (you read that correctly) has reached the California Supreme Court.

Apparently, the challenge is based on a claim to the effect that Prop 8 introduced a substantive change to the California constitution and therefore needed to be introduced by the legislature.

I have never lived in California and I do not know the law on the matter, so I am going to enlist the aid of some folks on this one.

Just a hunch: was there no legislative oversight in the process of getting the initiative placed?

This whole mess has raised a number of questions that go to the heart of what it means to live in a republic of laws, under a constitution; it makes me wonder what some folks think government by consent is.