Eric Marsden <emarsden(a)mail.dotcom.fr> writes in xemacs-beta(a)xemacs.org:
It is annoying to have to read (emacs-version) before being
able to bind something like the middle mouse button ([mouse-2] /
[button-2]).
(define-key something-map [mouse-2] 'some-function)
and
(define-key something-map [button2] 'some-function)
should both work. Do they not work for you?