The most plausible and empirical calculation I’ve seen yet suggests 210,000 Kindles in circulation!
Missing data point: how many modules did the manufacturer ship before the Kindle’s 11/19/07 launch?
Assuming 40,000 a month the rest of the year, which probably understates the 4th- quarter flow by quite a bit, we’d arrive at 530,000 or so by year’s end. I suspect by then the figure will be closer to a million.
Thanks to Jan at The Kindle Reader for sleuthing out the above-linked post!