Dev, i solely signed up to tell you what an absolute legend you are!
This problem has dumbfounded me for about six months and was driving me crazy. So many packets hitting the ISA firewall that I have the BES going through and filling up the logs and making them meaningless.
After making those changes exactly as you described, worked perfectly.
Are these switches documented anywhere? Google, Emails and phone calls to the appropriate support in RIM and our service provider yielded no results for me.