A post by James Bartholomew prompted me to go looking. In case you are interested, here it is:
This report shows that The Spirit Level has little claim to validity. Its evidence is weak, the analysis is superﬁcial and the theory is unsupported. The book’s growing inﬂuence threatens to contaminate an important area of political debate with wonky statistics and spurius correlations. The case for radical income redistribution is no more compelling now than it was before this book was published.
(There may of course be other rebuttals I'm unaware of).