Do we need utopias? There's been a nice debate on this between Norm, Matt C and Shuggy. I fear, however, that a couple of valuable distinctions (well, spectrums, actually, but I simplify) are missing.
Distinction one: utopias of people vs utopias of institutions.
Some utopians have seen the ideal society as one in which people's characters are different - for example when Thomas More praises the frugality and tolerance of his Utopians, or when William Morris claimed that crime could disappear. This strand of utopianism lives on, when our managerialist rulers claim to be trying to build a new culture.
Other utopians - for example James Harrington - have instead thought of utopia as set of institutions or rules.
The latter, surely, is more sensible.
Distinction two: utopia as a direct goal vs utopia as reflective equilibrium.
Efforts to build utopia directly have, as Shuggy points out, ended in the gulag. But this is not the only function of utopia. It can, more modestly, be thought of as an ideal against which to contrast our present imperfect world, as a reminder that politics can be more than the grubby pursuit of the illusion of power. This, I think, is the message of Norm's marvellous essay.