There is an excellent review of the state of knowledge in quantum spin liquids that was published in Science last week (Science, 5 Sept, 2008). It was written by Patrick Lee of MIT, and it covers not only the state of knowledge, but also a brief history of spin liquids and why it may be a new state of matter that we haven't seen before.
Note the role that high-Tc cuprates play in all this. It is just another example on why the problem of high-Tc cuprate superconductors isn't just an issue of superconductivity. I've mentioned this before that the physics behind the high-Tc superconductors is so rich that it touches almost every aspect of condensed matter physics. This is just one example of that point. It is why the quest for the mechanism of high-Tc superconductors is one of the most important problem in physics. It isn't just about superconductivity, but about a whole bunch of other related phenomena as well.