Economic determinism basically is a belief that social movements and beliefs are unmediated reflections of underlying economic structures. In its most sophisticated version, you get Charles Beard's "Economic Interpretation of the US Constitution" which attempted to explain the various clauses and subclauses in terms of the different economic interests of various constituencies of the American bourgeoisie. Now Beard was a Progressivist historian and didn't know any better. Althusser believed that it is a mistake to regard Marxism as a simple inversion of Hegel. If Hegel maintained that the dialectical unfolding of Ideas in history determine social relations and the state, then a simple view of Marx would tend to conclude that social relations determine ideas. Althusser is correct to point out that the relationship between social and economic relations is not "unmediated". Ideas, beliefs, customs, etc. become part of social relationships and can have as much material reality as a job or an apartment lease.
Althusser, interestingly enough, doesn't quote Marx as a counter- example to economic determinism. He cites Engels, that alleged "distorter" of Marx who, along with Lenin, has been the favorite whipping-boy of academic Marxists for most of the century, including the Rethinking Marxism gang, who purports to be Althusserites. Engels said, "The economic situation is the basis, but the various elements of the superstructure -- the political forms of the class struggle and its results: to wit constitutions established by the victorious class after a successful battle, etc., juridical forms, and then even the reflexes of all these actual struggles in the brains of the participants, political, juristic, philosophical theories, religious views and their further development into systems of dogmas -- also exercise their influence upon the course of the historical struggles, and in many cases preponderate in determining their form..."
Althusser, as I read him, was somebody who was urging the dogmatically minded--in his case, the French CP--to "go back" to Marx and Engels and forget about the one-dimensional malarkey coming from the pages of L'Humanitie. Nothing wrong with that.
Louis Proyect
(http://www.panix.com/~lnp3/marxism.html)