Michael Mann has a bash at Richard Muller at the Scientific American blog:
One might hope, however, that a scientist known for big ideas that didn’t stand the test of time might be more circumspect when it comes to his critiques of other scientists. Muller is on record accusing climate scientists at the University of East Anglia Climatic Research Unit of hiding data—a charge that was rejected in three separate investigations.
I think it would be fairer to say that two of the investigations didn't look at the subject and the one that did failed to report that they had found evidence of hiding data.