User defined command line switch no longer works

[mandar.mitra]

I have the following in my .emacs:

(add-to-list 'command-switch-alist '("diff" . command-line-diff))


(where command-line-diff is a function I've written). This used to
work well, but after moving to emacs 23.1 on Ubuntu 10.04, it no
longer does.

When I run

/usr/bin/emacs -geometry 155x47 -diff file1 file2

I get a message (in the Message buffer) saying:

command-line-1: Unknown option `-diff'

But command-switch-alist's value is (("diff" . command-line-diff)).

Am I doing something wrong? Has something changed since emacs 22? Or
is this some kind of a bug?

Thanks for any suggestions,
Mandar.