@Nick Fine - did you read this page? http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
"Text that is larger and has wider
character strokes is easier to read at
lower contrast. The contrast
requirement for larger text is
therefore lower. This allows authors
to use a wider range of color choices
for large text, which is helpful for
design of pages, particularly titles.
18 point text or 14 point bold text is
judged to be large enough to require a
lower contrast ratio. (See The
American Printing House for the Blind
Guidelines for Large Printing and The
Library of Congress Guidelines for
Large Print under Resources). "18
point" and "bold" can both have
different meanings in different fonts
but, except for very thin or unusual
fonts, they should be sufficient.
Since there are so many different
fonts, the general measures are used
and a note regarding fancy or thin
fonts is included."
Thought that was a pretty reasonable explanation, as for the size, I'd defer to the 'experts' as I have no access to alternative research that shows anything to the contrary.