There's been some discussion about this ruling on the internet, but I haven't seen any here.
California’s prisons are designed to house a population just under 80,000, but at the time of the decision under review the population was almost double that. The resulting conditions are the subject of two federal class actions.... Believing that a remedy for unconstitutional medical and mental health care could not be achieved without reducing overcrowding, the Coleman and Plata plaintiffs moved their respective District Courts to convene a three-judge court empowered by the Prison Litigation Reform Act of 1995 (PLRA) to order reductions in the prison population. The judges in both actions granted the request, and the cases were consolidated before a single three-judge court. After hearing testimony and making extensive findings of fact, the court ordered California to reduce its prison population to 137.5% of design capacity within two years. Finding that the prison population would have to be reduced if capacity could not be increased through new construction, the court ordered the State to formulate a compliance plan and submit it for court approval.
The SC (in an opinion written by Kennedy) affirmed the lower court, so they either need to fix the problem or release prisoners early so as to reduce overcrowding. There are two dissents, one written by Scalia, in which Thomas joined, and one written by Alito, in which Roberts joined.
Scalia made some bizarre comments in his dissent, the most notable in blog posts being the following:
Most of them [the prisoners who potentially could be released early in 2 years] will not be prisoners with medical conditions orsevere mental illness; and many will undoubtedly be fine physical specimens who have developed intimidating muscles pumping iron in the prison gym.
However, I think some of his other statements are actually more remarkable. For example, "One would think that, before allowing the decree of a federal district court to release 46,000 convicted felons, this Court would bend every effort to read the law in such a way as to avoid that outrageous result." Similarly, there are the arguments based on obvious fear of the result and arguments that suggest that judges cannot be competent to analyze evidence given by expert witnesses. The points about what these people might do are significant, since I don't see how such an argument wouldn't be generally applicable if it's relevant -- violation of civil rights? oh, well, admit the evidence (or allow the eavesdropping) anyway, since otherwise someone guilty might go free.
a more detailed discussion of the case, from The Monkey Cage, which focuses on the fear expressed by some (including the dissents) regarding the effect of the decision* and analyzes the reasonability of those assumptions.
*For example, “I fear that today’s decision will lead to a grim roster of victims” (Alito) and “terrible things are sure to happen as a consequence of this outrageous order” (Scalia).