I got a response from Winfield today.
For the last several months we have been going over
the 1400 figures, which were drafted in India. We're
editing them in Adobe Illustrator, and are 75% done.
Then, after a last pass on text edits, we'll be making
the index. This involves thinking of all the alternate
words blokes use to describe the same thing. About
that time CUP will start doing the page layout. Then
we'll have a galley proof to study and approve.
Looking on wikipedia Galley Proofs are limited runs used for proof reading and sometimes advance copies, so that should be the final step before mass publication.
That said it sounds like there's a minimum of months of work remaining, but it does sound like less than six months of work to my, untrained, ears, so I'd expect to hear a real release date in about five or six months or shortly after (since people are horrible at estimates and always underestimate everything).
I'd give a couple more months for reviewing the remaining figures, a few weeks for a last editing pass, and couple of months for the index, then a few weeks, maybe a month or two for proofreading. Then production would start and that would take a minimum of a few weeks to a couple months to build stock.
One article I found on Chronicle of Higher Education described hand building his index (completely after the fact) in a month. I'd expect some of this work to be done already and this to be a much large work, I'd expect two months there.