I've did a quick search but came up short. If a user goes to domain.com/webmail cpanel will bring up an authentication dialog box for user/pass. After logging in, they get to the cPanel webmail screen where they can choose which program to use. If you click on RC, it asks for the login details again.
Before I upgraded, it was working with -rc1. I had my tech originally install it, so they must have changed something to fix it. I'm not sure what it is.
Ask your tech and have him send us a patch!
This was the patch they used based on the popular tutorial on hostgeekz:
http://www.hostgeekz.com/files/hostgeekz/HGpatch-roundcube-NON-X-0.1-rc1
I would love to see this patched and built in. It seems like a big pain for me to do this with .1 stable.