I've bisected this problem down to the following commit;
unfortunately, there were some rather hairy interdependencies after
that point, so I wasn't able to test reverting it on top of master:
commit 93dbda7cbcd70a0bd1a99f39f44a9ccde8ab9040
Author: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
Date: Thu Feb 26 17:35:44 2009 -0800