Thanks for the LiveHTTPheaders suggestion - that helped a lot. It turns
out I was sending switch=Log%20In and I should have been sending
switch=Log+In

I'm now getting the response I was expecting - thanks again.

$ curl "-duser=bob&password=fish&switch=Log%20In" "-A Mozilla/4.0
(compatible; MSIE 6.0; Windows NT 5.1)"
"-k" "-L" "-I" "-v" "-b cookie.jar" "-c cookie.jar" -x proxyserver:8051
https://ftp.businessbanking.cibc.com/template/login

"THIS TRANSMISSION IS INTENDED ONLY FOR THE ADDRESSEE. IT MAY CONTAIN PRIVILEGED OR CONFIDENTIAL INFORMATION. ANY UNAUTHORIZED DISCLOSURE IS STRICTLY PROHIBITED. IF YOU HAVE RECEIVED THIS TRANSMISSION IN ERROR, PLEASE NOTIFY US IMMEDIATELY SO THAT WE MAY CORRECT OUR TRANSMISSION. PLEASE DESTROY THE ORIGINAL. THANK YOU."
Received on 2004-11-16