We're using version 1.7.16. We use ReviewBoard internally for our code reviews, and typically the review creator will assign several reviewers to the request. The review creator customarily expects and waits for all or at least a quorum of assigned reviewers to review and give a 'Ship It' before he feels comfortable committing the change, but sometimes reviewers are unavailable, otherwise busy, or don't feel they have sufficient expertise in the piece of code being reviewed to weigh in one way or another.
It would be nice for a reviewer to click a button to give the message of "sorry, I won't be able to review this request, please don't wait around for me to chime in". Maybe the button could be called "pass on review", or something like that.