In an attempt to kill two birds with one stone, I've written an
article for Boxes & Arrows, which will also appear in a week as a
Semantics article, but there's not much reason for you to wait for
that version.

http://www.boxesandarrows.com/archives/002595.php

Please comment! And please have patience with my erratic publishing
schedule. We're deep into crunch time on the 2nd edition of our
polar bear IA book.

Cheers!