I guess I would say the actual problem is that the forum is altering my text in a way that is:
a) unnecessary (since www works fine on its own)
b) not reflected in the rendering of the text in the preview/final submission until the next iteration, giving me a sort of "what you saw is what you get but you now don't see" (WYSIWYGBYNDS) sort of effect.
For what it's worth, my opinion is that it is useful to allow www links because they avoid clutter in both the edit text and the output rendering, but that's tangential to the bug, and I won't complain if the final decision is just to repair the disparity by "fixing" the immediate rendering.
I do recognise also that www is getting special treatment over myriad other URL forms that don't start that way, so in that sense it's not really "fair". Though that said, prepending 'http://' would still be a form of special treatment.
Anyway, I've submitted a report as advised at http://tracker.phpbb.com/browse/PHPBB3-10628
. Thanks for your responses. I hope I've cleared up exactly what the problem is.