Carlin Romano, Literary Critic at The Philadelphia Inquirer did a survey of book reviewers and came back with the following:
68.5 percent of book reviewers think anyone mentioned in a book’s acknowledgements should be barred from reviewing it.
64.9 percent think anyone who has written an unpaid blurb for a book should also be banned from writing a fuller review.
76.5 percent think it’s never ethical to review a book without reading the whole thing.
And 52 percent think it’s not okay for a book-review editor, in assigning books for review, to favor books by writers who also review regularly for that editor’s book section.
For the rest: CRITICAL MASS: Ethics in Book Reviewing Survey: The Results