i like what dot said here, and i would add that i think utpoian is often used in the place of idealist. I think perhaps often when anarchists are called utopian, at least part of the critique is aimed at idealism, that anarchists aspire to "higher" values, and implied in this i think is a proposition as to the content of "human nature", in this case being, that people are "greedy", "mean", "uncaring", "capitalist/authoritarian" by nature.
The critique leveled against anarchist utopianism, though, i think is partially true, because many anarchists i see are "utopian" in the sense that anarchy or communism is something that exists "somewhere else" in the future, not achievable until the glorious revolution wipes away all traces of authority and domination. There are, as i think should be obvious to many people here, proponents of a totally different conception of what anarchy and anarchism are, that it is lived now, that it is a way of being and relating to each other and to the world/society/authority, the former being informed by a set of values that we as individuals establish, the latter being a state of perpetual or permanent conflict against those. Bonnano's "anarchist tension" i think is a really good exploration of these things.