In an essay--tellingly published in The New Republic--denying that he's switched sides on Voter ID law (methinks he doth protest too much), Posner takes an unnecessary implicit potshot at Roberts:
... whatever judges may say self-servingly about being merely “umpires,” just calling balls and strikes and indifferent to who wins and who loses, their priors influence judicial decisions in areas in which the law is in flux because authoritative sources of law, such as the Constitution and statutes and precedents, do not yield clear answers to new legal questions.
A fair between the lines reading is that Posner is saying Roberts lied to get on the SCOTUS.
Posner has never been a conservative, but his journey from classical liberal to modern liberal (at best, liberaltarian) now seems complete. It's a pity we don't have formal excommunications anymore.